Diego Corrales...well, there has been a few, actually. You could also say Zab Judah, because of the adversity he faced, but still pulled it off...same thing goes with Hatton. But because it was a fight many thought Floyd would lose, and it was one of his first huge wins, I'll stick with Diego.
I'd say Corrales because I think he was the underdog in that fight but he totally outclassed Chico and that's where he really established himself.