I think he would've given anyone a run for their money:

Tyson - Ok, I think a prime Tyson would beat Klitch, but then again IMO I don't know if ANYONE would've coped with Tyson in his prime (including Ali).

Frazier/Marciano - Too small. If a 6'3" 210lb David Haye was too small, a 6' 205lb Frazier would struggle, and a 5'11 187lb Marciano would've been destroyed.

Foreman - Possibly yes. I think Klitch is a better athelete though and has overall better stamina but Big George was terrifying in his prime.

Dempsey - No chance. Dempsey was a brawler. He had no finesse at all. For all Tyson's ferocity, it was based in a rock solid foundation in boxing principles. Gene Tunney, a 6' 190lb ex-marine (and one of the forefathers of modern boxing) was able to totally out-think and outbox Dempsey. A 6'5" 240lb master tactician like Wlad would've destroyed Dempsey. Dempsey thrived in an era of brawlers and tough guys. Once Tunney and others like him started introducing an art and science to boxing, it hearlded the end for guys like Dempsey.

Back to 2011, it looks like it could be the last fight for Haye. There doesn't seem to be anyone left for the Klitschko brothers to fight at the moment.
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"Let your food be your medicine, and your medicine be your food" Hippocrates.