For me, it's a VERY close toss-up between the XY and DP seasons. If I had to choose... probably XY.
1.) I'm a sucker for good characterization, and it was done really well here. The main group had such a good dynamic and played off of each other nicely. I actually cried at the end knowing they'd split, and I haven't cried watching a Pokemon episode since Pikachu's Goodbye. Ash himself, imo, was the best he's ever been character-wise.
2.) The story was told very well, knew when to get serious, and ended up being kinda "shonen-esque" at times, which is something that I personally enjoy. The Team Flare arc was really awesome too. And dat Core Enforcer thoooo.
3.) Ash-Greninja. Before it showed up, I was wondering how they'd handle Mega Evolution with Ash. And I quite liked how it happens, it being the result of a bond beyond that which causes Mega Evolution, which suits Ash very well. And that theme that plays! I watch Pokemon dubbed, but his JP theme absolutely slaps. Kinda wish he didn't end up releasing him at the end tho.
4.) The Gym battles were really fun to watch. Some of my favorites included Korrina, Clemont, and Wulfric's battles.
5.) I'm not usually one to ship characters together, but holy eff did I enjoy the Amourshipping in this. Serena was so cute throughout the show trying to keep her feelings in, and everything from their "date", to her taking care of him while he was sick, to that kiss... Serena, pls come back, I'm rooting for you!
For s&g's, I'll include DP as well.
1.) Paul. He was more than just your everyday douche-y rival. Essentially Ash's antithesis, he represented everything Ash didn't believe. His character was written insanely well, opposing Ash and his ideals with his own at every point, leading up to what I personally believe was the best battle in the entire series.
2.) Ash's team was the best it has ever been. His team was so good, they had to create a character with a team full of legendaries just to make him lose the Sinnoh League.
3.) Characterization was done extremely well here too, with both the trainers and the Pokemon. Dawn's character development with contests, Ash's Turtwig essentially having to relearn how to battle using its bigger body after evolving, the list goes on. And don't even get me started on Infernape.
4.) Really great battles. Not only were they entertaining to watch, I really liked seeing Ash implement Dawn's contest moves into his own battle style, such as the Counter Shield. And even Dawn implemented aspects of Ash's battle style into her contests.
5.) Cynthia. That is all.