Garchomp is the one I'd vote for, if only because it's the one I've had the most negative experiences with facing in battle...and the most positive experiences with using. I'm not much of a competitive battler any more - to the contrary, all my knowledge on the subject is dated to the end of the fourth gen, haha - but even in the anime Garchomp is typically portrayed as all-powerful, whilst the others, not quite so much. Except for perhaps Salamence, given how vicious J's was.