From the best to worst;
1. SM - first series ever that made me like almost all of the characters and Pokémon, and that's all that matters to me.
2. OS - best comedy, lots of interesting characters and Pokémon but also some I didn't enjoy that much. Some of the Pokémon were forgetable, and it's not just because of lack of screentime, often PotD got more focus than main cast's mons.
3. AG - probably the best Pokégirl vs. Ash losing it's OS personality, no interesting rivals (except Harley), mediocre Brock and Max. It also started the trend of Ash tossing his all Pokémon barring Pikachu at Oak's.
4. DP - Dawn getting lots of focus, good rivals for Ash (Paul) and Dawn (Zoey). Nice villain arc. I liked how almost (?) all the game characters appeared, though I think stat trainers deserved better. Too bad Ash's old personality was almost gone and was turning into typical shonen hero I despise. It took a guy with 2 legendaries to kick him out of the league.
5. XY - Ash at his worst personality wise, way too OP (still not as ridiculously OP as his rival) and worshipped by everyone, especially the main cast. XY Ash was anything but underdog hero I enjoy the most. Bonnie was the most annoying main cast member in the show's history. Clemont was okay, better than AG-DP Brock, Tracey and Cilan but still not as good as he could've been. Serena was the best but I hated how her character revolved around Ash (and AmourShipping in general). I LOVED Team Flare arc, Sawyer was the great rival and Alain had interesting backstory.
6. Lots of wasted potential. I had a high hopes for Iris or Trip but they didn't live up to the expectations. Don George Tournaments were cool. I liked how they gave each main character a rival. It wasn't bad, but it didn't have anything to keep me invested for longer. The art style change added insult to the injury.