Hm? Manaphy and Shaymin don't even get a mention? That's sort of harsh. Anyways, for me, it'd be Mew. It's the original, after all, the ancestor of all Pokémon, the one that we had to use a glitch to obtain, the one that got a holographic card written in another language, the one that can learn all of the moves in the game. Celebi's pretty cool too. It can travel through time and it protects forests. Jirachi, meanwhile, came out of the blue for me and really doesn't have a right to exist, in my opinion.