I hate to say it, but Swoobat. He's just the cutest thing, and I was crazy enough to take him to the E4, even though Marshal's team had some Rock moves, I though he was fast enough to hit first and dodge them. Sadly, he wasn't. Nonetheless, he's still one of my favorites, and nowadays he flies me around post-E4 Unova.
Also, Panpour. I needed a Water-type though, so he was still on the team, but I replaced him with Tympole as soon as I had the chance.