Mosely would have been a great match up.
I'd say Mayweather probably has avoided a couple good fighters in his career, imo not because he thought they had a chance but because of $$$, other than that there really just isn't any "great" fighter out there in or around his weight class.
I've seen a lot of his fights, I have quite a few on DVD, and other than the Castillo fight, he just seems so far head of all his opponents on all levels, he destroys just about anyone, if not by power than with technique and speed to the point where it's embarrassing for the opponent.
The only thing I give Oscar is the weight advantage. I don't know how well PBF will do with the extra weight but honestly I don't see it having much effect.