I didn't think about Mew...that's a good one. I think I'd still like Pikachu though.A Mew, of course. :3 It's very cute and functional as is (bouncy bubbles? Teleportation? Telekineses? Heck yeahz), plus it can change into any other Pokemon.
I'd pick Mew. He's adorable and very playful, and he can transform into any other Pokemon, to boot.