If we're talkin about heavyweights, then it's gotta be Bonecrusher. He was a last minute substitute for Tubbs and he took advantage of an extremely undermotivated Weatherspoon. Despite this loss, I still can't figure why Tyson and Spoon never fought...
I would love for someone to beat Carlos Muassa as worst belt holder in the last decades. He was quite impressively poor for holding a belt (I dont mean it as a way to be disrespectful, Im just saying) If we are talking just heavyweights..... then Id say Valuev.
Valuev almost lost to a guy who Ruiz beat more then once when he was younger and better, and Ruiz gave a good fight against Valuev when Ruiz was clearly past his best. I really dont see how Ruiz is worse then Valuev
If we're talking heavyweights I think Maskaev is up there. Credit to him for his Rahman victories, but he was never any more than a contender Liakhovich and the Briggs that beat him were both weak champions too. Again, contenders. Ibragimov was a level above those guys, and it was a shame he retired after his first loss Povetkin is also a level above those guys. He has wins over Byrd (albeit a past prime Byrd- but it was Povetkin that exposed that), Chambers and past prime Chagaev. Time will tell how good he is I suppose Herbie Hide? Hasim Rahman?