I haven't gotten used to all the new pokemon yet, so I'm horribly old-fashioned when it comes down to this. Pichu, Pikachu and Raichu used to be my favourites, but after playing Pokemon Stadium oh-so-many years ago, I totally fell in love with Ivysaur, Rapidash and Dragonair also, for their elegantish and lovely looks, strong moves and Dragonair's usability. And Vaporeon, with it's unusually high HP (is that even true anymore?) and somewhat strong moves. I like other Eeveelutions also, and practically every water or electric type finds their way in my heart.
Lately I've also grown fond of Kirlia and Celebi, and who wouldn't have loved Mew after the first movie!