I really can't believe I'm saying this, but...
It seems like there are two kinds of boxing commentators. The Castites who under-rate Marciano because he was White, of course, and the rest, who over-rate him.
Good as Marciano was, likeable as he was, and as much acclaim as he deserves for never allowing himself to be defeated,he is NOT the greatest HW who ever lived. Jeffries and Dempsey were better. By leaps and bounds. Jeffries too, was never defeated in his real career-only when he let himself be talked into fighting Jack Johnson as an out of shape 37-old ex-fighter. He was never knocked down in his real career-excellent punchers actually broke their hands on the Jeffries chin! He was larger, stronger, and faster than Marciano, probably had the greatest stamina of ANY fighter from ANY era. He fought an opponent just about Marciano's equal in Tom Sharkey (that day and age produced more REAL fighters than we can imagine today). He was 21 solid pounds in the days before weight training was recognized as important for fighters, probably be 230 with a good regimen today. If he was alive today, he'd be HW champ in boxing, or, considering his wrestling talents, MMA.
Dempsey had all of Marciano's power and toughness, with much greater speed, and skill. Think a taller and heavier-handed Mike Tyson. Considering how he terrorized bigger men, he might also be the HW champion today, even facing the Russians.
Of course, since Marciano weighted between 185 and 190 lbs soaking wet, he could easily opt to reign over the cruiser or light heavy weights virtually unapposed today.