Roy Jones Jr., mainly because he's fallen so far. I'd like to be reminded of how physically talented he truly was.
When p4p Mayweather is already getting mentions for being the fastest ever and you realize Roy Jones Jr still is clearly faster than Mayweather and can put that speed in 1 second 8 hook combinations and carried power up to 200 lbs. (The reason Ruiz wasn't doing anything was because he was losing the firefights he had with Roy, he got "floored" plenty of times) End of round 4 Ruiz was in plenty of trouble if that was the round, he also got his nose broken. Roy is truly p4p the best ever, not the greatest but he's the best.
Going with the 24-27 "rule" Floyd Mayweather is in his prime from 2001 -2004. In that time frame he fought at 130, 135 and 140. Roy Jones prime is from 93-96. Jones fought at 160, 168 and 175. I think 130-140 is deeper today than 160-175. I would go with Mayweather for that reason.
Match Mayweather (prime or not) with anybody at the "deep" 140 division and you come up with a bunch of fights nobody wants NOW. Floyd becomes interesting at 147 vs: Tito DLH Forrest Quartey Mosley or even Whitaker