Mayweather, no doubt about it. And by far. This guy is so freaking good that it is hard to understand what's going on in the ring while he's fighting, even for the most experienced fans/trainers/fighters... This guy is several levels above any other fighter.
Yeah, agree on Ward, he was the other one I thought of apart from Hopkins. Very little physical tools to rely on, so for that level of success it really points to technical skill. The others may well have great technical ability but their physical gifts make it harder to tell where the one ends and the other begins.
I think Ward's speed and athleticism and especially footspeed are being slept on a little bit. He was obviously not elite in those areas but good to very good. He and Chad Dawson were throwing at the same time and Ward's were landing first, and Dawson had fast hands.
There is the best and then there's the rest. Out of twenty years there is only one fighter that stands above the rest and that is Floyd Mayweather Jr. Skills, ring IQ, speed, punch selection and delivery, respectable power and chin. He was an artist and a specialist. A cut above the rest.
If you're looking at non special guys (Loma, Floyd etc) then you're looking at someone like Mikey Garcia, does all the basics well, not elite speed or athleticism. Throw in a Canelo there too.