Dialga, if I wanted to make the timeline my *****, Biff Tannen style. And for you children that didn't get that reference, shame on you, and shame on your parents.
Otherwise, Leafeon. Specifically, the Leafeon I have in my team, because that thing is a beast. Every time it brushes off another flamethrower, I just wanna reach into the game and give it a hug. Besides, I wouldn't even have to feed a Leafeon; just let it sunbathe on the roof every day, give it plenty of water, and he's set.