Eric Aiken, IMO, is the worst titlist ever. Knocked out a faded Tim Austin in his last pro fight to earn the title shot against Valdemir Pereira, a weak titlist himself who barely squeaked out wins over Whyber Garcia and Emmanuel Lucero to be given a shot at a vacant title, and then beat some Thai guy whose only notable fight was a KO1 loss to Manny Pacquiao to win the title. Aiken won the title when Pereira was DQed for excessive low blows, although that was of the Anthony Peterson variety, where Pereira was throwing the low blows because Aiken was hurting him. Anyway, since winning the title, he's gone 0-4-1, losing badly to Robert Guerrero, Thomas Mashaba and Monty Meza Clay, drawing with journeyman Cruz Carbajal, and losing to some unknown prospect Todd Wilson. The other guy with a really solid argument is Juan Polo Perez. Went about 25-4 against Colombian cabdrivers before getting a title shot. Beat Eliyas Pical for the title, which is probably the only thing keeping him from being #1, but then went 19-40-2 for the rest of his career. Of "real" world champions, it's got to be Luis Santana. He was 38-14 when he got an unearned title shot against Terry Norris. Won the title because he stayed down after an illegal punch with a bad acting job. Defended the title, against Norris, when he was hit after the bell and Norris was DQed for a second time (this time legitimately). In their third fight, he got smashed in two. Never won a truly significant fight the rest of his career, though he had several wins over gatekeepers and prospects who flamed out. Other guys with solid arguments off the top - Carlos Maussa, Maselino Masoe, Kelvin Davis, I'm not counting the old WBO guys from before it was recognized, otherwise you get some really bad guys like Michael Bentt, Boone Pultz and Kamel Bou Ali. Anyone who says John Ruiz really knows **** about boxing. He might have been overrated by some, but overall he was probably an above-average titlist in this modern four-title era.
As for worst current titlist, I'd say it's Senchenko (undefeated but unproven, doesn't LOOK bad, but who the heck knows because he only has one decent opponent on his resume), Nathan Cleverly (who has potential to get good but clearly isn't there yet) or Pornsawan (who seems to have improved lately, but beat a shot Rachman for the title and lost wide decisions to guys like Sosa, Nietes and Sithsamerchai not long before then).