Not great but very good. There are a couple of fighters that maybe he should have fought but most of the "names" from his era will be on his resume'.
Straight A. Maybe A minus, just because of the gulf in quality between himself and most available competition in most of the divisions in which he campaigned. Even if he tied up all the supposed "loose ends" (Margarito, prime Cotto, Williams, Pacquiao - most of which, for the record, were after he already cemented his place in the HOF, long before he fought at WW) that wouldn't be enough to push him over the hump to be A+. You need to be that good and that lucky to end up with a truly A+ resume. There need to be a whole bunch of fellow ATG's playing in your sandbox. Like SRL for instance - he was that good and that lucky.
Among champs and other world class fighters on his era, it's one of the best. Among other ATGs, it's on the weak/poor side. It's an elite category and the standards are high.
:deal I honestly don't think Margarito would do anything for him. A prime Cotto would've been a little better but not much. Williams and Pacquiao would be/are impressive. Everyone talks about Margarito and a prime Cotto like Floyd would've losted. People forget, when Cotto was prime, Floyd was too. And Margarito is another "slow plodding Mexican". I don't think those fights would've done much for him.
If anybody but Floyd Mayweather had the resume Floyd had, people would be jumping for joy. Floyd will be a lot more appreciated in 10 years than he is in the moment.