Sullivan definitely. Arguably the most significant fighter in boxing history, and we don't really know whether he was a multi-skilled phenom or bar-room brawler.
Thanks, I wondered if it was real. Watching this I pondered if there was a write up of this fight depicting the two as swan-like footwork, perfect punching and the epitome of 2 great athletes, shoulder rolling, elusive combatants. Then to see the actual footage, bhwahaaaaahaa!!! Too funny to watch and take serious. Nevertheless good video. My dumb butt was looking for the volume...wait 1906....uh...nevermind.... My choice is Charley Burley. I've seen like 15 minutes of footage, where 8-10 minutes is slow-mo replay. Too many losses to think he was invincible, but too many turned him down to have just been another good fighter.
In a modern vein, Edgar "Mad Dog" Ross. #1 contender for the 154 lb title and barely a photo of him around, nevertheless actual film or tape.
Harry Greb hands down. if he's the only guy to beat Gene Tunney and he gave Jack Dempsey fits in sparring sessions he must have been something to behold. From what I've read he could spin you around while hitting you from every possible angle, all this while blind in one eye. The great light heavy champ Billy Conn was in awe of Greb's skills. Hell, Greb might have been the greatest ever!
Maxie Rosenbloom for me, though I'd expect he was probably boring beyond belief. Still really curious to see how he was so successful having apparently no power whatsoever. He managed a couple of wins over Heavyweights who were ranked at or around the time he beat them...Lou Nova (W10), Roscoe Toles (W10), and drew with Bob Pastor, and managed a top 10 ranking by Ring magazine in the late 30s...all of this near the end of a 250+ fight career. The Pastor and Nova results occurred within his last three fights (the other being a 3rd round KO over Al Ettore, which apparently was a dive). Impressive that he was able to pull this off that late into his career after so many fights. He must have been a defensive wizard. It's also a sign that the Heavyweight division probably stunk in the late 30s aside from Joe Louis.