Ali> Marciano
Many other fighters > Marciano

He was good but there were better fighters than him. He was the best in his time but wouldnt last vs later generation fighters like Holmes, Foreman, Lewis and Tyson.

Today ,he would lose to the Klitschkos , probably to Haye but beats guys like Arreola, Adamek, Povetkin.

Theres my 2 cents