Styles make fights
Very few fighters in history have had the ability to fight and succeed against any style. Floyd is one of them. That being said, he still beat Jose Luis Castillo in a controversial UD early in his career (most think he lost) at a time when Castillo was in his prime and Floyd was still approaching his. He also struggled against Maidana, albeit now on the other side of his prime. Both opponents had power and the stamina to apply constant pressure.
If we agree that his best weight is 147 and then ask who (if any) would be able to beat him if both fighters were to meet in their primes, this would be my short list of guys that have fought post 1980:
Sugar Ray Leonard
Tommy Hearns
Vernon Forrest
Paul Williams
Jose Luis Castillo
Terence Crawford
Oscar De La Hoya
All of them have certain things in common...size, an ability to throw punches in combinations and consistently for 12 rounds, and power. I did not list Maidana because while I give him a ton of credit (he continually threw overhand rights that would make Butterbean blush) and gave everything he had. But Floyd was past his prime-if both fought in their prime I think Floyd has the legs to stay out of harm's way and make it a boring victory.
Of the fighters listed, I think all pose legitimate threats and each bring a different strength to the table that would force Floyd to adapt:
Sugar Ray: Insane speed
Hearns: Power w/length
Forrest: Length + ability to attack/counter with power
Williams: Ridiculous length + limitless stamina (100+ punches per round for 12 easily) AND a southpaw
Castillo: 12 round power
Crawford: Jack of all trades, power, stamina
Oscar: Size + world class jab
Floyd vs. SRL
-This would have controversial decision written all over it. Both guys are so smart that they would be playing high speed chess for 12 rounds. Trying to set traps for the other, etc. It would probably have some exciting moments but the average fan would find it boring.
Floyd vs. Hearns
-Floyd would have his hands full for the first 5-6 rounds. If Shane Mosley could catch him twice at almost 40 years old, a prime Hearns would have a very good chance to test Floyd's chin. Ultimately I think he would take a UD or MD vs Hearns
Floyd vs. Forrest
-This would be interesting for sure. For as bad as Vernon looked (and completely lost) vs Mayorga...go back to the first thing I said "styles make fights". Both guys are world class boxers, so I dont see Vernon getting confused in this, and I think he would have the length and smarts to make it interesting. I'd lean Floyd but as with SRL this one might end up in controversy.
Floyd vs. Williams
-You can all say I am crazy. This fight would be fun as **** to watch because Paul Williams would give Floyd problems with his length and volume of punches, causing him to fall into an early hole. We would finally get to see him have to dig deep and come forward, either looking for the KO or at minimum looking to put Paul on the canvas to try and make up the gap. Unless he catches him he loses a UD. No shot he would have EVER taken this fight. And I don't consider Floyd to be a dodger or cherry picker at all. He fought basically everyone (except Manny) and was no more or less smart about his matchmaking than any other champion. Very few fighters go against conventional wisdom and are willing to fight anyone, anywhere.
Floyd vs. Castillo
-If you saw the second fight between these two I think it would be closer to what happens if both are in there prime. While Castillo could give him a challenge I would have Floyd by UD, coasting in the final rounds.
Floyd vs. Crawford
-Crawford would find a way to make this fight interesting. No way he let's Floyd make it a boring fight. He has always shown an ability to adjust on the fly...and he almost looks as comfortable fighting southpaw than he does conventional at times. I think Floyd would take a UD or MD, but it would be close.
Floyd vs. Oscar
-We definitely got to see a prime Floyd when they matched up, but Oscar was clearly past his prime. In his prime, Oscar would give Floyd just as much of a challenge with his jab as he did when they actually fought. But just as he did so many times in his career, how could you not expect him to inexplicably stop throwing his jab and take his foot off the gas? If the guy that fought Vargas shows up, all bets are off. Otherwise, I'd take Floyd by UD or MD.