In the current era of four major belts and diluted power among the sanctioning bodies, some pretty crummy fighters have won (and sometimes even hung onto) belts. If a belt is only as strong as its champion, then here are a few guys that the ABCs should be ashamed they let even get into a position to win a belt.
Luis Santana. Santana had lost 7 of his last 10 fights when he got his opportunity for titles and the lineal championship at 154 against Terrible Terry Norris. He won the title by faking getting knocked out by a punch to the back of the head. This was similar to what Francisco Lorenzo was able to pull off against Humberto Soto, only this time the sanctioning body didn't say they wouldn't award the belt. Then, he managed to actually defend his titles in a rematch with Norris by constantly jawing at him, until Norris got so ticked off that he actually decked Santana between rounds, getting disqualified again. At about the 55 second mark of this video, you can see exactly how Santana both won, and defended, his belts:
Outside of Norris, his best win was against Shawn O'Sullivan, a protected Canadian fighter. He faced many of the best of his era, and he literally lost to all of them. He retired with a career record of 42-17-2, including the two disqualification wins that he's most famous for.
Eric Aiken. "Mighty Mouse" Eric Aiken had a 15-4 career record when he got his title shot. Included among those four losses was a loss to 6-5 Leo Martinez, 2-3-1 Antonio Oliveiros, 5-0 Johnny Edwards and Al Seeger. Both Edwards and Seeger were subsequently destroyed in one round by Yuriorkis Gamboa. Aiken got his title shot when he was a late replacement in a title eliminator against Tim Austin, who got knocked out by Aiken while ahead on the cards and subsequently retired. Aiken then won his title by defeating then-undefeated Valdemir Pereira, who was disqualified for repeated low blows, and who also seems to have retired after that fight. Pereira himself had only won the title by beating a Thai fighter with a puffed up record (who had lost all of his previous step-up fights) for a vacant title. Since winning the title, Aiken has gone 0-3-1. Granted that has mostly been against good competition, but the difference in class was obvious even when he was facing guys like Thomas Mashaba and Monte Meza-Clay, and his draw was against journeyman Cruz Carbajal.
Carlos Maussa. Sometimes one punch is all it takes to win a title. Maussa is a wild mauler, and there's really no better way to describe it. Immediately prior to getting his title shot, he lost to Arturo Morua, who has gone 6-8 since fighting Maussa. Even though he was a late replacement, the Harris-Maussa fight probably never should have been sanctioned as a title bout to begin with. On paper, it was an obvious mismatch, and that mismatch carried forward into the ring for the first 5 or so rounds. However, Vivian Harris melted down before our very eyes, losing all of his stamina, and Maussa took advantage, knocking Harris out on a wild hook. His next fight, Maussa was dominated by Ricky Hatton. Then he was beaten soundly by fringe contender Manuel Garncia, and then knocked out in one by Victor Ortiz.
Michael Bentt. When someone tries to tell me that I should count Joe Calzaghe's WBO title defenses from the '90's as major victories, this is about as far as I usually need to go to point out that the WBO title WASN'T a major belt back in the '90's. Bentt was a top amateur with a bit of bad luck early on. He actually lost his pro debut to a nobody, claiming that he lost because nobody told him he was facing a southpaw. After nine easy touches (best opponent: 14-10-1 Mark Wills), Bentt was brought in to be an easy opponent for Tommy Morrison, who was trying to set up a big money fight with Lennox Lewis. Bentt sparked him quickly.
After that fight, Bentt next faced Herbie Hide, who ended Bentt's career with a one-sided seven round knockout beatdown. These days, Bentt is an actor, having played Sonny Liston in the movie "Ali", and a big chunk of why he gets the work is because he can plausibly claim to be a former heavyweight champion of the world.
Some other candidates you may know and love (or hate): Leon Spinks (narrowly beat Righetti for a shot, then beat a very old Ali, but lost every other significant fight of his career and ended with a record of 26-17-3); Gavin Rees (Kotelnik exposed how extraordinarily limited he was); Alex Arthur (who won his belt while drinking in a pub because Joan Guzman was stripped, and proceeded to give a speech as if he had just gone 12 hard rounds to win the title); and James "Bonecrusher" Smith (finished his career 44-17-1; had Chazz Witherspoon's number, but everyone else had his).
Feel free to add your own in the comments.