I am pretty much unbeatable in Soul Calibur II with Link. Really only my elder cousin would be able to beat me. I get demolished in Soul Calibur III, though.
I am pretty much unstoppable with Roy in Super Smash Bros. Melee. I've rarely lost to a human opponent, but it pisses me off when people act like the Cubs just won the World Series in (however many years it has been) for the first time. Made me want to punt my GCN off a rooftop.
I'm also literally unstoppable with Goku in Dragon Ball Z: Budokai 3. Then again, Goku is only slightly less omnipotent than God in that game. I can do very well with Bardock, assuming the computer isn't a cheap punk and ring-outs me (which is why the World Tournament stage, in my opinion, should never be used outside of World Tournament mode).