Michael Watson vs undefeated power puncher (22-0 22 kos) and future 2 weight champion Nigel Benn in a terrific contest.
Assuming that by championship you mean the world championship, Sam Langford has a few candidates, from beating Joe Gans whilst just a teenager to multiple wins each over Harry Wills, Joe Jeanette and Sam McVea. Charley Burley beating Archie Moore deserves a shout too, as do Packey Mcfarland's wins over Jack Britton and Freddie Welsh.
Andy "Hawk" Price beat future World champions Carlos Palomino (undefeated at the time and on 3 days short notice+flooring him) and Pipino Cuevas. What is worse is that Cuevas would win his World title 6 weeks after the loss to Price whilst Palomino would come to England to shock John H. Stracey 20 days after Price beat Pipino meaning he held clear victories over the 2 reigning Welter champions in the World. Price never got a world title shot. His biggest fight was against Harold Weston (coming off a draw with a undefeated Benitez) which he lost on points. Weston got 2 title shots. His other big fight was a visit to England against the courageous but hittable Dave "boy" Green. They had a terrrific fight with Price almost stopping Green who rallied to eke a decision. Green would later get an undeserved title shot against Leonard whilst Price would also fight Leonard (2 months before challenging Benitez) for the NABF title and get destroyed in a round.
Good call on Andy Price — the 147-pound ‘peoples’ champion’ for a bit — but I have to say the Leonard fight was the Hawk’s biggest. World stage, iirc Ray’s tuneup for Benitez … if Andy had been able to upset that apple cart surely he’s fighting for a world title next. Beating Pipino and Palomino in that span is quite an accomplishment.
James Scott in a span of 14 months in 1978/79 dominated two No. 1 light heavyweight contenders: Eddie Mustafa Muhammad and Yaqui Lopez. Beat Eddie pillar to post over 12 rounds and beat up Yaqui over 10. Not sure either were dominated like this in their prime years by anyone else. James put on a body punching clinic in both outings. Alas, Mr. Scott was incarcerated in Rahway prison in New Jersey and the powers that be (rightfully) determined he couldn’t be a mandatory contender if he couldn’t find a way to get out for a title fight — they weren’t going to compel a champion to go into prison to fight him, and if he won that would cause further headaches since any challenger would basically have to accept any purse on offer to go behind bars to challenge him as there really couldn’t be any competitive bid situation or a way for promoters to make offers for him to take the title on the road. A plan to get Scott a furlough to fight in Atlantic City fell apart when it was denied and he never rose above No. 2 in the rankings and never got a shot.
Always felt sorry for Price, scored 2 terrific victories but at the wrong time and will always be remember for being destroyed by Leonard. I´m pretty certain Palomino would have loved to erase that loss but Price lost to Green and Weston. Cuevas possibly not. The sad thing is that reigning WBA champ Angel Espada was ringside for Price-Cuevas and after seeing Andy outbox him demanded an easy title defence against Cuevas the following month. Pipino crushed Espada viciously in 2 as an 18 year old to become champion whilst Price would lose his next two bouts. Boxing is a tough and illogical sport!
I love that performance, and the electricity of those UK fight crowds. I thought it was in some ways like Ali-Foreman, with quite a bit of the “rope a dope” being in the middle of the ring. Benn would load up and throw these bombs and catch mostly gloves. Watson would just come back with quick 1-2s and land much more cleanly. SB Nation wrote a really good, long article on him about 10 years ago. https://www.sbnation.com/longform/2014/3/12/5496096/james-scott-jailhouse-boxer-profile