My definite favorites would be Cilan, Elesa and Skyla (Jap. Names; Dento, Kamitsure and Fuuro). Not only do they challenge you with three of my favorite Pokémon types--Grass, Flying and Electric, but their designs itself are just amazing. n_n It's hard to choose out of the three for my favorite as a whole, but to judge on the kind of challenge they put on, I wouldn't be able to do so yet.