Kanto: the battles weren't that great, the league was atrocious, and Misty and Brock could have used more episodes.
Orange islands: Tracy could have done more.
Johto: way too many fillers, Brock and Misty were mostly background characters.
Hoenn: bad fillers, May started out slow.
Battle Frontier: some fillers were bad, Brock started to get sidelined more.
DP: bad fillers, Team Rocket, Brock became stale.
BW: Ash getting reset, TR vs TP cancellation, the league and the rest of BW season 2, Cilan was just DP Brock with a more over energetic personality.
XY: Bad fillers, Ash overglorifying season, the entire Korrina arc, Serena's goal starting out slow and then being rushed at the end, Clemont being pushed into the background again.
Kanto: Ash losing because of Charizard's disobedience. Ash releasing many of his fully evolved pokemon.
Orange Islands: Brock staying at Ivy's lab. Tracy's team should have been more developed.
Johto: Ash having older pokemon on his team when he should focus on Johto pokemon only.
Hoenn: Team Rocket still around when we have Team Magma/Aqua as replacements for the region.
Battle Frontier: Brock needed some episodes.
Sinnoh: Team Galactic should be the villains of the region and not team rocket. Brock needed more development for his team.
Unova: Team Plasma didn't really happen much in the region. Charizard returning when Ash could have developed his other pokemon. Losing to Cameron at the league. Annoying catchphrase being used.
Kalos: Serena having a crush instead of focusing more on showcases. Ash releasing Goodra. Team Flare not being the main villains of the series. Clemont needed episodes for him and his pokemon.
Kanto - Pacing was a little too fast, battles were lackluster, and the league ref. Just that ref.
Johto - Pacing was horrendous, Misty and Brock felt shoved in the background at times, GS ball being wasted story-telling potential, character exaggeration of TRio
Hoenn - When fillers were bad, they were bad. TM and TA were not used much. Lack of Ash rivals.
Sinnoh - Dawn had too many rivals and not enough screen time, Brock was pointless, TRio was worse, Ash is stale at this point
Unova - Gym battles were lackluster, Ash is an idiot, and Team Plasma was generic
Kalos - Rivals were lackluster, TRio are somehow more pointless, Serena wasted a year of episodes find a goal we could have guessed was related to fashion and performing, Clemont had nothing to do after his arc but grow stale, League is off to a terrible start.
* Kanto: A lot of its flaws can be justified since it was the original series. The writers/animators were 'finding their footing', so to speak. Some of its 'flaws', such as Ash earning a bunch of badges in unconventional ways or the anticlimatic loss at the league with Charizard's disobedience (although both of these points highlighted his rookie faults), weren't ever repeated. Never seeing Pidgeot and Primeape again sucked.
* Orange Islands: Tracey was an unnecessary third wheel. Releasing Lapras.
* Johto: Wasted potential with the GS ball. Dull middle stretch. Obvious executive meddling and disagreements over the direction of the show led to things such as the Ash x Misty subplot built from Kanto/Orange Islands to fizzle away (though it did show up time to time). Misty and Brock could have had more individual focus. Cyndaquil didn't evolve, even though it put up so much work. Johto captures deserved some more glory at the league.
* Hoenn: Not the finest fillers, that's for sure. Team Rocket had clearly overstayed their welcome by this point. More could have been done with Team Aqua/Magma. League should have been 1 or 2 episodes longer, and Ash vs Tyson deserved to be the semifinal (why repeat another Top 8 finish).
* Battle Frontier: Expected more appearances from Misty and Tracey. Obvious writing decision to make Ash a momentary idiot so that Charizard gets screwed over and loses against Brandon, to create 'dramatic tension' of how Ash was going to win without his ace.
* DP:: Brock was an unnecessary third wheel. Team Rocket. Ash's team soon became the Chimchar/Infernape Show. Suicide Mountain killed Torterra. Ash x Dawn should have happened in second half of the series - everything was there for it. Dawn's rivals could have been more interesting. Ash v Trollbias should have been a two-parter, so that way Swellow wouldn't have gotten P.I.S.-screwed over.
* BW: One of the most tragic decisions was to "reset" Ash back to Kanto/Johto levels, in terms of both personality and as a trainer. At the same, trying to fit this "reset" Ash with continuity to previous series. BW could have been a good series if it was simply another protagonist instead of Ash. Iris tried to be a Misty clone, but not as endearing. TP vs TR being cancelled, and episode N TP being disappointing. Charizard's reappearance was utterly pointless, and the Charizard vs Reshiram tease never happened. Cameron might be the worst character in the anime's history, and for Ash to lose 4-6 and regressing back to the top 8 felt like a slap in the face from the writers. Way too much Oshawott, and a bunch of neglected others on Ash's team. DA! was terrible. Yeah, pretty much everything in Season 2 was an utter trainwreck.
* XY(&Z): Serena's goal was delayed for a year, when there was no reason for it. SatoSere's retconned introduction, and not going anywhere with it, especially from Ash's side, since the beginning. Clemont's goal and development fizzled away, and there was no Lumiose City villain arc before his gym - not to mention, he was one of the only gym leaders who felt like an underdog to Ash, who just styled upon him as if he was an E4. Leprechaun should never have become Ash's rival - or he should have showed up at the beginning of the series as a slightly less experienced trainer than Ash who wanted to catch up to him, not as a complete newbie who becomes a super rookie with a Cameron-esque death squad in like 50 episodes. Serena's rivals were also poor, and to call them "rivals" and not as fodder is being nice. Showcases became boring and a worse version of Contests. The league skipping over all the rounds leading up to the semifinals was disappointing, when all it would have needed is one extra episode full of clips showing his progress. One of the most disappointing things is the complete self-contained nature of XY, and its lack of continuity to OS-DP, even though XY Ash felt like a post-DP Ash.
I'll skip Sinnoh since it's been a while since I've watched it, but as for BW and XY.
-The league was poorly handled.
-Bianca, Team Plasma and other elements were poorly adapted.
-All of the rivals were poorly developed and terribly executed.
-Most of Ash's team were poorly developed.
-Iris' development was bad, Axew remained weak and a lot of her accomplishments came to her too easily.
-Season 2 in general really.
-The rivals are still poorly developed and executed, with Trevor actually being worse then any of BW's.
-Poorly paced development for Noivern, Goodra, and other Pokemon and characters.
-Clemont's goal and development fizzling out as his character ultimately went nowhere and he faded into the background. Luxray also felt superfluous as it's role was basically to just be a quick fix to Clemont's underwhelming team. His match was also lackluster, as it felt more like he was playing from behind then challenging Ash.
-Serena taking too long to get to her goal, and said goal was pretty uninteresting compared to Contests.
-The league matches up to the semis have been underwhelming.
Not watched all of these in full but from what I remember:
* Early instalment weirdness (though some was enjoyable).
* Lots of filler, especially by the time of Johto.
* Brock and Misty became increasingly less active.
* I remember a lot of the Pokemon or one shot characters being a bit bland or annoying. Some of the twerps' stories were a bit dull until TR caused trouble as well.
* Team Rocket got flanderized into such mindless losers that their treatment bordered on painful rather than funny.
* In contrast a lot of alternative villains either felt over pivoted and rarely got a satisfying comeuppance or climax.
* Went on a bit long.
* Probably least developed twerp dynamic.
* TR got semi flanderized again, they at least were a BIT more dynamic and could get do more than just butt monkey humour this time. The obnoxious mindless behaviour however got upped a thousand times, especially in the dub.
* Again went on too long, especially since it didn't even change regions like the previous two series.
* Ash's reset.
* When the catharsis of them challenging Ash for once faded, serious Team Rocket were actually kinda boring as characters.
* Twerp companions were maybe a bit too obnoxious, perhaps trying too hard to compensate for lack of TR humour.
* Start of the dub truly downplaying the use of original BGM.
* Arcs were downplayed and cut short in favour of repetitive filler.
* Serena's goals felt half baked and her accomplishments usually the result of plot armour than her own hard work.
* TR, though arguably their best compromise characterisation wise, are usually stuck doing the same schemes and reduced to filler.
* Ash despite some great progress in his arc, feels his dullest character wise.
* Team Flare are coming...don't worry...and they'll be like a billion of them...and they'll be huge...very soon...
* Arguably the most half assed editing the dub has suffered. More scenes ruined by unenthusiastic acting and bad music, and title cards and other bells and whistles lazily edited.
-The battles weren't very well animated and had a lot of 1 hit KOs
-The League was below average
-Lacking in any good COTD episodes
DIAMOND & PEARL
-Bad COTD episodes
-Ash losing at the League to a trainer with Legendaries
-Ash and Pikachu were nerfed
-Trip was too skilled for a beginning trainer
-Bad COTD episodes
-Ash losing to a less competent trainer who only brought 5 pokemon to the League.
I can't recall any of them exactly anymore, so i just do BW and XY.
Ash and Pikachu were nerfed
Annoying Ash and Iris
Annoying side rivals (Burgundy and Georgia)
The story went pretty bad after the title change
No more Meyer backstory and revelation
Team Rocket is pretty boring
James only having one pokemon
Tierno being shoved aside due to Shota being over glorified.
Well this is fun. I would rewatch Kanto-Johto constantly as a kid while Unova and Kalos are the most fresh in my memory. I guess I'll only cover those.
Kanto: Nearly all of Ash's badges were handed to him. Ash released Pigeot and never came back for it. Ash released Primeape and never came back for it. Ash never caught that Haunter. Then there's the whole vs Ritchie match in the league. I don't mind Ash losing to Ritchie because of Charizard, but the referee was trash tier.
Orange Islands: Just give Ash his dang Celebi already. The extra Ash/Misty moments felt pointless seeing the sudden decision to have it go nowhere in Johto. Tracey was okay...I guess.
Johto: Too many fillers. This was also a very forgettable region for me. It's actually the region I remember the least despite watching it over 20 times.
Unova: Ash's reset. And Decolora Adventures. Combine the two, and you get Ash disbanding their group over an apple. Also, the over-used "gag" of nearly all of Iris's Pokemon disobeying her at one point just emphasized how little she was growing through the entire saga. Good job, champ. Last 1/3 of BW was the writers scrapping up fanservice + Charizard when they realized the series was going to shet.
Kalos: Pokemon Showcases being introduced just so Serena would have something to do and be good at. Sorry but, no. Mega Charizard and Mega Sceptile being given to rivals as an obvious reason to not give them to Ash. I'll withhold my opinion on the league until it finishes.
Indigo League: There was only 1 good battle in this season. I understand Ash was a rookie, but it didn't feel like he matured enough as both a person and trainer in this season. Ash losing the league made the most sense here, but the entire tournament felt so un-epic that I just couldn't feel for his loss.
Orange Islands: Nothing really stood out, and Tracey is just not a good member of the team. He's so, blah. Drake was not impressive in the slightest.
Johto: The pacing was just atrocious, with way too much filler and not enough plot related material. Gary was under utilized this season, which took alot of wind out Ash and Gary's confrontation in the league. Brock and Misty didn't do much.
Advanced: Same issues as Johto, but turned up even further. 2nd worst league, with Ash lacking strong rivals (writers clearly focused a ton more on May's). Dub related criticism, but the new voice actors during the voice switch were like nails in a chalkboard to listen to.
Diamond and Pearl: Dawn's peppy attitude can be annoying to sit through, and the pacing during the later poritions of the series is all kinds of messed up. A great league was ruined by the a**pull to end all a**pulls. Aka, Tobias.
Best Wishes: Ash's character reset. Iris' annoying disposition. Cameron. The dropped Rocket vs Plasma subplot (understand something horrible had happen to hurt that, but it should have still been revisted). All the rivals are dismal with the exception of Stephan. Cameron. N was under utlized. Cameron. Charizard and Dawn were nothing but fanservice, with their presence amounting to nothing. Cameron. Oshawott's attempts to be Piplup 2.0 fall flat. Cameron. Ash team is quite under-developed. Cameron.
XY: The Amour ship teases are obnoxious. Pokemon shouldn't be about the love life of 2 kids. Bonnie can be annoying at times. The series in general started off very slow, like Ash having no real rivals for a good while, Serena having no real goals for too long, and Clemont and Bonnie not having much development most of the time. Team Rocket are at their worst this season, lacking the seriousness from Best Wishes, yet also not possessing the charm the once had in the previous series.
I feel too biased towards Kanto to make flaw lists on the other series. But there are a few big flaws of the OG
* A lot of the gym badges were won not through battling
* They played fast and loose with typing, though this is still an issue somewhat
* The releasing of Pokemon. Not with Butterfree, but with Primeape and Haunter. It comes off as they weren't sure what to do with them, so just gave them up
Absoultley nothing about it was a retcon because we knew so little about Ash's childhood to begin with.
OS: Shitty battles and Johto was terrible, Misty and Brock being background once Johto got started, in fact most things wrong with OS come from Johto
AG: Ash/May's relationship kinda fizzled out when Drew came along, Ash had no rivals (but unlike XY his character was a lot more entertaining to make up for it) and the League was bland cakes.
DP: Fillers are pretty bad on average, Dawn could've changed a bit more personality wise, Brock was pretty meh but I preferred him here then in AG, Urusula not being treated like crap couldve been nice to, Ambipoom release was awful, and Ash shouldve won the league.
BW: A many myraid of things that many people have gone over and I dont feel like getting into
XY: I dont care for Amour, fillers tend to be unimpressive, Showcases dont rock my world, some rushed arcs, Clemont's arc concluding and not having anything else to contribute, most of the rivals kinda suck and are a non-threat (but I do disagree with the peolpe who put Sawyer in that category, I think he's decent), Alan's character motivations being written around Ash once they met/him at the league, TR can get really aggravating.
I don't really recall much from the series before Sinnoh (though it took a while getting used to the new voices going into Battle Frontier) so I will just list what I know from Sinnoh onwards:
-Team Rocket were a bit annoying
-Brock felt a bit stale
-Tobias just appears out of nowhere in order to avoid an underwhelming league finish following Ash vs Paul, also felt that he had something missing compared to Richie, Harrison and Tyson.
-Ash reduced to a beginner, the Elesa battle been one of the most facepalm worthy moments.
-No noteworthy rivals apart from Stephan, Trip became annoying after a while.
-Very underwhelming league, notably with the Cameron battle
-Zekrom appearing before Ash was never really explained in the end.
-Too many Pokemon for Ash means most got "thrown under the bus" in terms of character development.
Kalos (based on experience before the XY & Z series)
-Ash basically having next to no rivals until Swayer appeared.
-Showcases felt a bit meh compared to Contests.
-Serena's rivals were not as interesting as May's or Dawn's.
Rewatched up to the Ash v Gary league a couple year ago and they got extremely repetitive. Lame side plot, fillers, etc. Ash v Gary was great
DP: didn't really watch. Ash v Paul and Ash v Tobias were great although Tobias was one of the negatives for that series
BW: Didn't really watch either but Trip was like a poor man's Gary, Cameron, and the Ash/Pikachu nerf was dumb
XY: First season I'm watching completely since AG/BF. Glad they restored Ash to his normal development path. Ash Greninja and Mega evolution were great and brought me back to the Pokemon/the series (at least temporarily). Apparently these writers wrote Origins, and I can see that. Just like Origins was one massive Charizard circle jerk, XY has basically become one massive Greninja circle jerk. Not terrible because the Ash-Greninja thing is interesting but it's becoming pretty clear that Greninja is the emphasis (Ash v Sawyer in e34 feels like one giant build up for Greninja v M Sceptile.. hoping Ash v Alain is more battling along the lines of Ash v Paul/Gary). Things also feel a little rushed. I'm hoping that's because the writers are new and never wrote an entire season before or because Nintendo surprised them with an earlier than expected announcement of SM