1. AG/DP
2. XY
3. OS (Kanto and Orange Islands arcs would be tied with XY; Johto arc would be between SM and BW)
4. SM
5. BW
I haven't watched enough episodes of JN (only seen 1/3-1/2 of the series) to offer it a fair ranking.
1. AG and DP are my favorites because I felt I grew up with Ash during these two series. Ash became a better battler and a more 'mature' individual, while still retaining some of his entertaining snarkiness and fiery passion from OS. The AG team is my favorite regional squad - they each had a personality, and they were all dawgs, ready to thrown down in a fight and cover for each other when somebody couldn't deliver. The balance and lack of any true ace, until Sceptile very late in the series, was refreshing. I know the show moved onto fewer full battles in more detail/episodes, but I still liked how jam-packed the Hoenn League turned out. The battles were still entertaining, and everybody got to shine. Heck, people want to talk about the Sinnoh, Kalos, or Alola teams, but Sceptile/Swellow/Glalie/Corphish/Torkoal are right up there competing for that #2 regional team spot (JN is obviously #1). Swellow is one of my favorite Pokemon. I loved how he was basically AG Ash as a Pokemon - uttery tenacious. It's a shame they let it get KO'd in that fashion against Latios, when it really should have gotten at least 1 or 2 hits (one of the flaws of the Sinnoh League). The Battle Frontier was awesome, perhaps even my favorite arc, because Ash used his reserves and he truly began to distinguish himself as an elite trainer. Charizard completing his arc by beating Articuno was wonderful, and seeing the OG Gang of Four defeat Brandon was memorable and showed how far Ash had come.
Moving onto DP, the rivalry with Paul was obviously great, as widely considered now. Infernape's arc wasn't necessarily as good as Charizard's IMO, but it was still incredibly satisfying to watch it prove Paul wrong in the end. The Ash vs Paul Sinnoh League battle is the best battle in the entire show, and I think that's a fairly common sentiment. My only gripe with it, as well as Infernape's arc, is that it overshadowed the rest of the team. Torterra got shafted with the Worf Effect, and Rock Climb was probably the dumbest, most suicidal move ever created. In general, I appreciated how the battles became so strategic in DP, reflecting Ash's own growth as a trainer. Moreover, the Sinnoh League might just be the best league conference in the show altogether.
Overall, in AG and DP, the show itself took strides to improve from OS. Yes, it lost out on the zany humor and the Ash/Misty/Brock interactions, but it was enjoyable watching May and Dawn have their own character arcs, rivalries, and goals. I thought it was good to see May discover her interests and ultimately choose Contests, with Ash being somewhat of a mentor figure. Contests were a bit rough-around-the-edges in AG, but I liked May's rivals (particularly her romantic subplot/rivalry with Drew). All of these ideas were refined in DP. Dawn was a true co-star and I liked how much she bounced off Ash. Honestly, Brock didn't need to be here, and I felt he could have been written out after AG. Thus, AG and DP was 'peak Pokémon anime' to me.
2. XY is the best 'standalone' series. It had limited direct references of continuity to prior sagas, though to be fair, there were lots of indirect callbacks to AG. It's probably the series I'd recommend to anybody interested in watching the anime in 2023 because of how self-contained it turned out, and for its overall quality. XY Ash was cool, heroic, and practically inspirational, but I missed his funnier side and banter with other characters that he had in other sagas. The battles were amazing and well-animated, with great fluidity and 'camerawork.' The Ash vs Olympia double battle was a particular highlight. Amourshipping drew in a lot of people, and while Ash x Serena had some nice scenes together (at the camp; in the snow), things didn't really go anywhere until Serena finally confessed in her literal last scene. The Mega Evolution Specials were epic, especially Act III. Team Flare actually received a full-fledged villain arc, though the whole Giant Rock was a bit out there. This series somewhat frustrated me a bit, because it had a lot of missed opportunities (evil Malamar never reappearing, no Clemont and Blaziken Mask-focused episodes, Amourshipping being stagnant for much of the series, limited continuity references/no reserves at the league [though that may have been intentional], Sawyer showing up way too late to build up that master vs student rivalry, Alain not being nearly enough of a 'enemy team' rival), that could have elevated this series even further. Serena also received the Mary Sue treatment, which could have been easily avoidable with better writing. The Kalos League should have been longer, and it felt rushed, as if Ash had no competition until he faced Sawyer and Alain. However, Ash vs Sawyer was a great full battle, with plenty of firepower mixed with strategy, even if it was completely unrealistic that Sawyer became that good in that little of a period. Ash vs Alain was also solid, but c'mon, Ash-Greninja should have beaten Mega Charizard X. The best part of XY is that I felt it reinvigorated the fanbase. Maybe that's just my own personal opinion, but after what happened in BW, I think there was just a lot of excitement seeing Ash get the respect that he deserves. Ash's placement in the Kalos League finals and his overall skill level set the stage for SM and JN to make him a Champion, and later the Monarch.
3. OS is iconic and hilarious. Everything was brand new at the time. The show trialed a lot of concepts that would later materialize in more depth in subsequent series. Therefore, it's easier for me to forgive some of the flaws of this saga. Ash started off as the cocky, snarky, clueless trainer from Pallet Town who got pity badges, but showed flashes of brilliance. Ash/Misty/Brock had wonderfully amusing interactions, and remain one of the best groups (if not the best) in the entire anime to date. Pokeshipping was absolutely a thing, and you'd have to be willfully ignorant to not think there wasn't anything building up between Ash x Misty in Kanto and especially Orange Islands, culminating in the second Pokemon movie. The Kanto and Orange Islands arcs were just so zany, and it's easy to feel nostalgic about this period. The Orange League challenges were very unique, and obviously Ash's full battle victory against Drake (and his Dragonite) was iconic and really Ash's first major accomplishment. As good as these first two arcs were however, OS went downhill in Johto, where it became apparent that Ash's journey was nowhere close to being finished, and Misty was going to be written out (RIP Pokeshipping). The series became a slog of POTD/COTD episodes. Ash and Gary should have battled and competed against each other more to set up for their Johto League full battle. Luckily, Master Quest and the Johto League salvaged this region. Bulbasaur vs Meganium and Charizard's epic carry versus Gary and Harrison were the highlights. Overall, Charizard's overall arc across Kanto/Orange Islands/Johto is my favorite of any of Ash's Pokemon.
4. SM is....interesting. The vibe is so much different compared to the traveling badge quests of prior sagas. It's a tropical fun, lighthearted, slice-of-life, 'Ohana means family' type of story. I actually enjoyed the concept of a school serving as the main hub, with adventures around the islands. I've said this many times before, but SM turned Ash into a concept. As in, due to the relaxed tonality of this series, they redesigned Ash to fit the softer and rounder art style, and made his personality more animated and goofy, even flanderized at times. Just like how it is hard to reconcile BW Ash after DP Ash, it is hard (albeit to a lesser extent) to imagine XY Ash acting that way in SM. Then again, SM Ash wasn't written to be the cool, confident, and inspirational type, and have friends who were always looking up to him (or crushing hard in Serena's case). Rather, the classmates each had their own goals and interests, and Ash just fit among them. At the same time, SM Ash wasn't necessarily a worse battler per se - he actually retained a lot of his prior saga skills. I want to add that I don't necessarily agree with these fluctuating personality changes. OS -> DP had an imperfect, but apparent character arc, and every subsequent series just zigzagged Ash's personality. I felt it took a long time to get things actually going in SM, but the pace eventually picked up with Lillie's arc and the Ultra Beasts. There weren't a lot of captures due to the larger cast, but each character and their primary Pokémon all got quality screentime. The show also reintroduced continuity with the return of Misty and Brock, which was a pleasant surprise. There's a lot of hullabaloo over the Alola League, evident in another thread on this forum, but I thought it fit Alola. While it wasn't my favorite league, I appreciated that they took their time by giving ample episodes unlike prior sagas, and they finally gave Ash the W. Moreover, they followed-up with an arguable top 5 full battle in the show in Ash vs Kukui, where Ash gloriously justifies his title as Champion. All in all, while SM wasn't my favorite series, I can see the definite appeal to it, and why so many people do enjoy it.
5. Oh poor BW. I'll admit that if BW was the start of a brand new series featuring a different rookie protagonist, then it would have actually been a fairly decent opening saga. However, it's impossible to view BW without considering what it did to Ash. The infamous 'reset' of both Ash's skill and personality back to OS stage, essentially set the anime back 3 years. I can forgive a rookie trainer to somehow expect a single Pokémon to sweep a gym leader, or that embarrassing 5 v 6 loss to Cameron (who himself is such a BS character), but not a post-DP Ash. Trip was also just a poor man's Paul. There's really not a whole lot to be said, as BW has been maligned for this 'reset.' I liked that this series did try to add more elements. Team Rocket were more serious, and the loss of the Team Rocket vs Team Plasma episodes hurt. The Club Battles were great, and the host of recurring battling rivals were great. Ash/Iris/Cilan had an enjoyable dynamic. Honestly, BW Season 1 was not bad if you could somehow imagine that it was an alternative universe Ash on screen haha. However, I felt the series got worse afterwards. Ash just caught too many Pokemon for everyone to get ample focus. I wish they accentuated the struggles of a constantly rotating composition, and justified that is the reason why his team was so underpowered at the Unova League. I've explained in another thread why Ash vs Cameron should have been reframed as Ash being the recipient of a DEM evolutionary power boost, and the loss being viewed as an upset. Charizard's return was just bait. He didn't really do much, and the much-hyped Charizard vs Reshiram battle never materialized. Episode N was decent in the end. If BW was meant to have several OS callbacks, then DA was supposed to be akin to OI. But oh boy, what a dull ending and waste of 18 some episodes that could have been used for the league or Episode N or literally anything else more eventful.