In terms of mortals, Goku is almost undefeatable. Possibly. I wonder why we've stuck with only anime and Manga and not ventured into the world of Marvel and DC. For one thing... Doomsday can't be defeated in the same way twice, once beaten, given some time, he can't be beaten that way again. Eventually he'll be immune to everything anyone throws at him.
Or even other series of heroes. Can't think of that many at the moment.
Hey, did you know there's going to be a live action version of DragonBall? It's called... well, Dragonball.