Assuming every one is on their best night. Leonard or Pernell take it. I'd give a slight edge to SRL but damn Whitaker was a freakin Master.
Don Curry - he had the destructive power the other three lack. Mayweather punches like a ***** compared to Curry. Curry at his peak when he fought Colin Jones and Milton McCory was a awesome fighter Lloyd Honeyghan beat him yes and he was never the same but he was more electric than the others could have ko'd them all. Pernell would probaly have done the best just run like a ***** from Currys power all night long.
for me it comes down to pernell vs. leonard. curry was through the roof talentwise but never proved it in the ring against true elites. mayweather is still not on the level of whitaker or leonard and would lose to both head to head. leonard has the power and versatility that whitaker lacks but he wouldn't lay a hand on him in his prime. In a boring fight that is mercilessly booed, I'd have whitaker shading it in the finals against leonard.
There's no way Leonard loses to any of those fighters. Whitaker is on the level of Leonard, but not as a welter.
****, thinking about it it's a much tougher call at that weight. we never saw what whitaker could really do at welter but we know leonard was incredible. whitaker's best weight was 135...i have trouble picking against whitaker but it's looking more like a toss up.