OS: The humor. It's fun, weird and bold without being obnoxious. The dub really helped here because it added some solid, sarcastic jokes.
AG: Probably had the most entertaining o Contest stories. May and her rivals bounced off each other so well, and the whole Contest set-up became one of my favorite storylines.
DP: Battle strategies.
BW: Group dynamics. Most groups felt horribly unbalanced, but Ash/Cilan/Iris all had great dynamics with each other and as a group. Itis and Cilan were sidekicks, but they both got a good amount of focus and had a lot of work put into fleshing them out.
XY: Fingernails, I miss them. But aside from that, they had the most developed villain storyline this anime has ever seen. It's weird how they took one of the worst villain stories in the franchise, and just course-corrected them into something really good.
SM: Best supporting cast. We had a lot of recurring faces, with their own storirs and connections to the main cast. Alola feels tight-knit and familial because of it.
JN: Continuity, especially regarding past characters. I don't think this should even be a feature of JN, this is how the anime SHOULD handle its past characters from now on. On top of that, all these returning characters, and even Ash, are treated with such respect.
EDIT: My kids say that Journeys has the best Pokemon cries. They're obsessed with UNO LA LA LA.