Unova. It plays out as a boring children's program entirely lacking in interesting characters and comedy. Overall, I despise it and cannot fathom how anyone could like it.
I also dislike Hoenn and the Battle Frontier. While I like May and much prefer her over Misty, I still find Contests to be a drag. They play out as overglorified pet shows lacking in zest and excitement. I also felt the rivals (both Ash and May's) were aggravating, Team Rocket and Brock were by and large poorly utilized, and the Team Aqua/Team Magma storyline was weak and uninspiring.
I found Sinnoh to be a disappointment, too. Dawn was a May clone, Team Rocket and Brock were still poorly used the vast majority of the time, and many of the rivals (both Ash and Dawn's) were insufferable characters. The only upside I can think of is that it introduced Conway, Lyra, Khorey, and Angie, all of whom were entertaining characters who brought life to the show.