I'm going with Frazier. Power wise, he was close to both of those guys, and he can throw it all night, in addition to wearing the opponent down.
In terms of execution, Frazier's was no doubt the best. In terms of raw power - Tua. Pound for pound power - Dempsey. But is what I just said an answer?
Dempsey had a great left hook but also a great right hand. Frazier was known for the hook although it was more of an accumulation but the hook he landed against Ali in the 11 and the 15th were up there. Dempsey had more one punch KO power with his hook but he did not rely on the hook alone. Frazier had a short prime but in the FOTC he fought a prime Ali, so I give Frazier the edge in quality of opponent. Tua is not in the class of the 2 men above in as far as the quality of men he KO'd a young Ruiz, Oleg and Rahman not the greatest chins
Two fighters who might deserve a mention in this thread are Sam Langford and Sam McVea. Harry Wills was knocked out twice by his ridiculously deep body of opposition (excluding his last fight) and both times Sam Langford did it with a single left hook. Sam McVea was known to double, triple and even quintruple his left hook.
Definately either Dempsey or Frazier - lean toward Dempsey for blazing speed and deadliness (if thats a word?) - see the hook that put Willard down for the first knockdown - personally think Dempsey's famed left hook was one of (if not the) best in heavyweight history although Fraziers pretty spectacular too - my fave is the last one he finished Bob Foster with - that was truly brutal - Tua is not even in the same class as Jack and Joe though sorry
Tua had more one punch power than dempsey and frazier. Fraziers ko's came more as a result of an accummulation of hooks. Dempsey comes in at a close 2nd.
Jack Dempsey threw wide punches all the time, out of those three fighters. In terms of accuracy, power, and damage. Honest David Tua edges it, just on share power alone. But Mike Tyson is better than all three, also Evander Holyfield's left hook is greatly underrated. I remember in the first Riddock Bowe fight, when Holyfield went left hook crazy. WOW..