My favorite is Ampharos, and I have about zero justification for it. From everything I can tell, it's pretty useless for battling (though I'm still going to try), and it has a sad, sad move pool. But I just love the thing. It's one pokemon family I've consistently used since G/S.
I also like Alakazam. In the single player, at least, that's a pokemon that will never let you down.
Oh, and turtwig.