Which defining fight did Floyd have to prove how good he was?

Calzaghe was thought to have been fading and too old after his faultless performance against Lacy to be a threat against the big strong powerfull Kessler, yet he proved how good he is by figuring him out, eating up his best shots, he really proved his worth especially at his age and how long hed been at the top.

Floyd doesnt have that icing on the cake