I did a thread of this some years ago, same subject. I think the majority agreed that the 1971 Ali would have fell victim to a Marciano victory. But the 1960's Ali would have cut Marciano to ribbons, and one could argue the Ali of the '72-74 could have beaten Ali as well (strange as that sounds).
Myself you cant compare Marciano to Frazier, any more than you can compare Dempsey to Jeffries, and Jeffries to Tyson, etc. Frazier was busier and bent forward when he bobbed; Marciano just bobbed down without bending forward. Dempsey weaved more than bobbed; Jeffries crouched and then leaped at his opponents, etc.
I believe Marciano would have won a close decision over Ali. However, in a series, its hard to say. Marciano was a volume puncher while Frazier wasnt. Joe started slow and built up as time went on; Marciano started fast and kept up that same pace throughout. Dempsey started off fast, slowed in the middle rounds, and then came back to destroy you. Jeffries always stayed at one pace. Etc. etc. etc.
See where we get off in a tangent?
Now the greatest fight of all time? How about Marciano against the peak Joe Louis!