Tua is the obvious answer. I mean, his name stands out so much when the subject is brought up that it's a struggle to think of anyone else. He was unfortunate that he came along at a time when there were only a handful of world titles to go for. Cooper is another who has to be up there. I'd throw in the likes of Brian London, Dick Richardson and Joe "Max" Bygraves as other unlucky heavyweights from that era.
Jim Driscoll warrants (honorable) mention, since his claim at featherweight was superceded by Abe Atell's. Jimmy Bivens hasn't been mentioned yet and he should.