Denny O'Neil, Neal Adams, Terry Austin and Dick Giordano pit Superman against Muhammad Ali in All New Collectors' Edition C-56 (1978). It's not a bad story, although at the time it was released I thought the whole concept was stupid and didn't read it.
The story begins as Clark Kent, Lois Lane and Jimmy Olsen all head out to interview Muhammad Ali. As they do so, an evil alien shows up, threatening to destroy Earth unless a champion from the planet defeats their guy in a boxing match.
Superman is assuming the champion is meant to be him, but Ali takes offensive at this assumption, pointing out that he is actually from Earth, and a more skilled fighter. The alien is kind of amused, and decides that the pair of them should fight to determine which one gets to be the champion.
So Superman winds up taking boxing lessons from Ali. They only have 24 hours to train, so Superman takes them into a realm where time moves slowly. It also is under a red sun, so Superman has no super strength or powers. The page impresses me, to be honest. Had never thought so much about the strategy of boxing.
Then Ali and Superman get pulled out of this realm by the aliens, who feel that they are cheating by giving themselves extra time. They meet the alien challenger, and Ali plays his egotistical game, explaining to Superman that it's all part of his psychological strategy for winning.
The match winds up drawing a massive intergalactic crowd, and is broadcast throughout the universe. Adam Strange and Alanna get a cameo, watching from Rann. This story falls between their appearances in Justice League the previous year, and Showcase a couple months down the road.
To the surprise of Lois Lane, among others, Ali beats Superman soundly, and then goes on to face the alien. Except that is not quite what really happened. Superman lost intentionally, to allow himself to go after the alien war fleet while Ali is in the ring.
So Superman gets a bunch of great sci fi space battle action, rather than just lying around injured for the climax.
But it's still Muhammad Ali who gets to steal the show, defeating the alien boxer.
Ok, so it's not the greatest story ever told, it's still so much better than something called Superman vs Muhammad Ali was likely to be.