Unova definitely. By far the worst season IMO. Don't make Ash an incompetent trainer, get rid of Iris completely and replace her with the BW/BW2 female player character, keep Team Rocket as competent villains but give them their old personalities back, don't make Bianca such a klutz, introduce Team Plasma much earlier and make them have a much more important role similar to what DP did with Team Galactic, give Ash some decent rivals (Cameron was the absolute worst), scrap the Decolora Islands saga and make the main saga last longer, and for the love of Arceus DON'T bring Charizard back. I'm sick to death of seeing it. I don't care how popular it is, he's had his screen time. Time to move on.
The previous seasons were okay except the end of DP. I'd change that so that Ash would either lose to Paul or a different trainer altogether instead of Tobias and his Darkrai, who I'd remove from the anime entirely. I prefer legendary Pokemon not being used by trainers. Noland using Articuno was okay as it didn't belong to him.
For XY I'd remove any hints of Amour shipping and give Serena a decent goal. Showcases are extremely boring and, to me, are a really poor replacement for contests which I used to really enjoy and I'd like to see Serena develop more as a character without it. I'd also get rid of Bonnie because she brings nothing to the main cast. I wouldn't have had Ash release Goodra, Jessie would have gotten Noibat along with Pumpkaboo, James would get a Phantump to go with his Inkay (I always prefer characters I like getting the Pokemon I like, I don't like Ash) and they'd be less frequent but actually do something relevant when they did appear. The lack of Team Flare in the main anime is also a letdown. They should have appeared much sooner. It's BW having no Team Plasma all over again.
Lastly, in general, I'd make it so some older characters reappear occasionally. Namely some of my favourites or previous members of the main cast, most notably Dawn and May. I miss them a lot.