Probably his fight with Ndou. It showed all of Mayweather's skills in one fight,his defensive and offensive. The way he was rolling with Ndous' punches on the ropes was amazing,some of his punches were so dam fast and the finish,like 4 rapid fire right hands-WOW!

A close second would be the JLC fight.I thoroughly enjoyed that fight,and have seen it about 10 times.It was Castillo at his peak and man he was awkward.Still Mayweather's toughest fight by far imo. Far tougher than Hatton or Oscar.

The Corrales fight is up there too.