I very much like the new legendary forms that were introduced. I like both Kyurem formes, although despite preferring Zekrom to Reshiram, I prefer White Kyurem to Black Kyurem.
I also love the Therian formes, it's nice to have them look so different to one another (as opposed to their regular form where they're basically sprite edits of one another). Out of them I like Landorus the most, since he's all.. beast-like and whatnot. Then it's a tie between Thundurus and Tornadus. I don't really know what Tornadus is supposed to be, but yeah, it's a tie between the two. :)