I think on these "worst opponent" threads, we need to specify if it's over the WHOLE career or just title defenses or what. I mean Jean Pierre Coopman could probably beat Tunney Hunsaker.
I think this sort of thread should be taken from when a fighter gets established at the top. As has been mentioned,low level guys are part and parcel of an up and coming prospect's education. Ali fought his share of 'tomato cans' like anyone else but overall,his resume was better than any other heavyweight.
I will say Brian London who stated in a interview i got in position to fight Ali but i was not good enough to. Though Lubbers, Coopman and others are contenders for the dubious title of worst.
Ali's resume is the business, he can't be fighting Joe Frazier's all the time. The fellow will be remembered till the end of mankind!
I'm not sure that Coopman and Dunn would beat Hunsaker, becasue they both had EARLY KO losses to even lesser fighters. Ali's weakest challengers were (starting from the worst) Coopman, Dunn and Brian London. Lubbers wasn't title fight.
During Ali's title reigns, it was Jean Pierre Coopman. If we're talking his entire boxing career, it was definitely Ali's fourth opponent, Jimmy Robinson (who was a late sub). If we're talking exhibition bouts, too, Kenny Jay (the wrestler) was awful even by Midwestern A.W.A. wrestling circuit standards. Kenny Jay's wrestling nickname was "The Very Capable" Kenny Jay. LOL Not very intimidating.
Brian London shouldn't be among the worst. He had some very good wins - against Zora Folley, Joe Erskine, Willie Pastrano, Billy Walker, Pete Rademacher, Tom McNeeley and Amos Johnson (who was set to fight Ali until London won). And Richard Dunn, for all his faults, beat Roy "Tiger" Williams (no easy task) and had wins over nearly all the top "domestic" names in the U.K. (until he faced Joe Bugner after Ali). Coopman wasn't much. Although, there's video of him fighting when he was 53 years old on YouTube (against Freddy De Kerpel) and ... at 53 ... he aged better than nearly all the other fighters from the 70s.