This thought has only just came to me, so please don't think this is my definitive/comprehensive list or anything as I am well aware that I will have missed many great answers to this question. If taking a 'great' to mean (for the purpose of this discussion) someone good enough to be in the top 10 of a weight division in h2h/ability terms, who can you think of that was good enough in another division to be considered a 'great' there as well as in their own natural/optimum weight division? For the purpose of this discussion, best to think of only the traditional/original 8 weight divisions. For example: I think the Duran who beat Sugar Ray Leonard was definitely good enough to be considered a top 10 welterweight of all-time h2h, despite being a natural lightweight. I would rule out Ezzard Charles (atg light-heavy h2h, not top 10 heavy OAT IMO). Other examples I can think of just now: Sugar Ray Robinson (great middleweight, natural welter) Henry Armstrong (great light & welter, natural feather) Alexis Arguello (great light, natural feather) Roy Jones Jr (great lightheavy, natural middle) Who else? What about Barney Ross? Pernell Whitaker?
sam langford- hard to pinpoint what his natural weight would be by todays weight classes but he has a solid set of wins at light heavy and heavyweight. I think he won most his against heavyweights weighing around 160ish so I'd probably say he'd be around middle/ super middle