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.
Jack Dempsey easily demolished 250 lbs. 6'6" Jess Willar hardly breaking a sweat. The only heavyweight I've ever see with eaqually skillfull head movement was Tyson and MAYBE Frazier. True he lost twice to a very artful boxer in Gene Tunney but they were decision losses and Gene Tunney was no less skilled, though much small then Klitch I'll grant you that. As for Marciano and Frazier, well Marciano hit incredibly hard for his size, had a chin of stone and knew how to stifle tall, logn armed and skillful boxers. Frazier, meh Klitch might be a tough fight for even a prime Frazier, it's hard to say. Foreman would have hospitalised Klitch if he was in his prime, Ali was much better then Klitch at the same kind of style. Foreman was bigger, had a way better record and Klitch lost to much lesser opponents. And I think you're mistaking Dempsey's unique style as bad boxing technique when it was just a different approach. He beat some real good boxers, you win some you lose some. He was far from just a brawler I think few boxing historians woudl agree that he had no technique, he was a master of infighting and bobbing and weaving.