Since we're now apparently talking about anime, I can post mine~!
While Unova is my favourite region when it comes to the games, the same isn't true for the anime. The show just lost all of its charm after Sinnoh, with Brock finally permanently leaving (he seemed to do this after Johto, but he reappeared in Hoenn after a few episodes and remained throughout it and Sinnoh) and the episode titles losing their punny charm.
Not to mention that Dawn was far cuter than Iris ever could be (and had a better personality too), and Cilan...meh. Oh, and Ash once again left all his older Pokémon with Oak, and Pikachu loses to a freaking Snivy, of all things (sure, Pikachu was affected by Zekrom, but still. This is the same rodent who defeated Regice during the Battle Frontier arc, not to mention finished off a Darkrai - and the Snivy was a brand-new starter, having zero experience. HOW).