A prime Ali and a prime Marciano?
I think the best reference is the Marciano-Moore fight. Marciano was fighting an Archie that was way past his prime, yet... oh well, the fight is classified as a classic.
While Archie is considered as one of history's biggest punchers (141 career KOs, the most by any fighter in history) and Ali is not, Archie, when he met Marciano, was pretty old and has had tons of fights at Lheavy where he dominated for 9 years and 2 months-- the longest reign in that division and 2nd only to Joe Louis' over-all. His meeting with Marciano was his 176th fight! He was floored by Marciano 5 times, including the last in the 9th when the fight ended. But he got to Marciano too! Marciano went down in the 2nd.
That Moore fight was Rocky's 49th consecutive win, without a single loss, and was his last.
Between Rocky and Ali? I'll pick Ali.