Louis would destroy Vitali. Different class of fighter. Prime Ali vs Prime Dempsey would be my choice.
I've seen the more obvious ones play out in my head a million times already so i'd go for something that's gaurenteed to entertain, have a few beers and enjoy the night. Don't wanna risk Ali vs Louis being a stinker for example. Maybe Duran vs Mosley.
Honeyghan v Mayweather jr. Call me stupid but i think a peak Honeyghan would give Floyd all manner of grief. Not saying he wins but Jr. would know he was in a fight.
There are several but if I gotta pick just one it would be Roberto Duran {1976} vs Ike Williams {1946} at 135lbs. The two most dangerous slugger/boxers in Lightweight history.
If we're picking one fight, the fact Greb has no known extant fights, and on this forum is a consensus top five PFP, makes a fight involving him hard to pick over. Having picked Greb, the logical opponent is Robinson, although I would be happy with Monzon or Hagler. My 2nd pick would be Duran vs Benny Leonard, as we have only one fight fight on film of prime Benny and Arcel favored Leonard over Duran. I would be ecstatic to see Mayweather replace Leonard though, and this fight would probably be the one I personally would look forward to the most. Coming in at third place would be any two of the trio of Armstrong, Pryor, and Chavez facing off against each other.
Just one fight? Fights like Frazier-Tyson, Hearns-Robinson or Trinidad-Mayweather would be too obvious. I'll go completely outside the box and pick John L. Sullivan v Bartley Gorman.