I would consider Archie Moore to have been in his prime when he fought Marciano. Throughout the 50's, he only lost to three men, Harold Johnson, Floyd Patterson and Rocky Marciano. Everyone else he pretty much dominated.
Moore had won 44 of his previous 45 fights, coming into the fight with Marciano, mostly against top competition (including the top ranked heavyweights). After losing to Marciano, he reigned as light heavyweight champion for 7 more years.
This was one of Moore's last fights prior to facing Marciano:
He hardly showed any signs of being washed up, KOing Johnson in the 14th round with lightning fast combinations.