You're looking here for the best win under MoQ of the century - although in reality, there aren't that many years under consideration because of when the rules were written, 1865ish. You want the best fighter that was cleanly beaten while in his prime, i'd suggest, but you define the meaning - what is the best win of the 1800s?
I think Dixon was a lot more accomplished at that point than Gans was. I know this is unpopular, but I think Dixon may well have been better than Gans overall too.
I don't see why, Dixon has a lot of quality wins, fought on dead even terms with Young Griffo, despite conceeding significant weight, when Griffo was probably the best lightweight in the world, fantastic longevity being a top operator for about a decade. Frank Erne ranked Dixon better, for what it's worth. I don't think there's that big a gap between them.
I don't think it would be a big gap, no, not at all. But I think i'd insist on a class. I think Gans is the greatest champion in history. He's the greater fighter for this century, though. What other wins would you name? Seems to be just you and I for a moment, name another fight that you think really mattered from the 1800s, see if we can put a little list together.
Hmm. . . In no order a few others I'd consider up there Fitzsimmons beating James J Corbett Kid McCoy beating Tommy Ryan Jack Dempsey beating Dominick McCaffrey I'm sure I'm missing some though.
Maybe, I think it depends how much you count the manner of victory, he did have a pretty decent weight advantage too. Very good win, but I think you'd have to put it alongside his losses to Kid Lavigne and Mysterious Billy Smith.
Tom Spring vs. Jack Langan January 7, 1824 For the Heavyweight Championship SuperFight 1 (BQ - Before Queensberry) (click on the image to expand) https://ibb.co/BGnwRjW
Fitz over Dempsey and Corbett, Jackson over Slavin, McCoy over Ryan, McAuliffe v Myer, Griffo's "loss" to McAuliffe,
Luis Ortiz win over Sonny Liston in 1867 Is one of the best wins in the sport albeit even though both were a touch past prime.
