I say Tyson because his reign of terror lasted the longest. Even after he lost to Douglas, his opponents were still afraid of him. People never forgot the visuals of him easily brutalizing everyone during the '80s.
George Foreman was a terror too but he wasn't feared very long when you think about it. Few gave him a chance before he upset Joe Frazier, and he dropped the belt to Ali just a year later. After he was KO'd by Ali, his mystique and confidence were gone. Guys felt as if they knew how to beat him now. Ron Lyle almost did, and Jimmy Young actually succeeded.
Liston also suffered from the Ali curse, although initially everyone thought Clay's victory was a fluke and made excuses for Liston. After Ali confirmed he was the better fighter in the rematch, no one was afraid of Liston anymore.