The only slick boxer that beats Pacman is Floyd, even then I believe the fight to be really close and not just a foregone conclusion.

Raheem and Campbell are just some of the names thrown around that would beat Pac.
Zab Judah was also mentioned (not here but other places) that would beat him. Alexander before his questionable win against Kotelnik was another guy touted as being able to beat him also. And who can forget Bradley, the guy people said Pac ducked to fight the lineal champ at 140 and to make 13 million.