His speed, obviously.
He's fast, he's relatively powerful, and he's just awesome.
Well, his speed, for one. And he's a much better ninja bug than Ninjask in my opinion.
He beat Ghetsis' Hydreigon in my brother's White
His smug look in the sprite just looks awesome(I base my liking on mons by design,not by stats)
Her(mine is female) speed, Ninja-ness, and "I don't give a ****" artwork!