I've thought about it and (besides Pwill) there's also Clottey. No one has ever beaten him definitively. His losses are only 1 disqualification (where he was ahead), and two close bouts with Cotto and Margarito. No one has ever looked good against him. A very underrated fighter IMO.. although with that being said, he also doesn't have a very big draw. Hate to say it but I think the road probably ends with MW/Pac, I cant see either of them wanting to fight anyone risky (other than maybe the winner of Mosley/Berto), as they'll never see anywhere near this kind of payday again.
Floyd said it (to Pac) a long time ago:
"All roads lead to Mayweather."