The answer is obvious: Goku
That's the almost unanymous answer you will get from any fanbase, it is an idea that has been well engraved in our pop culture.
And it is no wonder, since Dragon Ball is loosely based on the myth of Superman.
So, the only person that would beat Superman, is his manga counterpart?
This webcomic was done by Joel Ojeda Lopez from Deviantart,it can be read here, on his site:
http://mistermoster.deviantart.com/gallery/27844038
One thing I loved is the action from the very first frame, perspirating action everywhere, and how the entire fight was so well presented and epic, that it has absolutely nothing to envy to the fight scenes in Bleach.
The creator certainly established his own realm of style to create a world in which Superman and Goku can coexist without the clashing of their aesthectic origins.
There's always the risk that Superman would be unrecognizable in manga style, or that Goku looks awful if drawn in classic Western style. Ojeda Lopez managed to create an interesting mix of both.
I'm not going to spoil how the fight turned out, but here's a key panel that'll make you wonder:
 |
| Kamehameha vs. Heat Vision????? |