Can't believe no-one has mentioned Lorien yet! The most stupid character concept in any sci-fi EVER. The oldest being in the galaxy who was just sort of hanging around waiting for someone to talk to? Everything he said and did was complete BS. "Do you have anything worth living for?" "Yes I kind of LIKE LIVING!!!". Come on, Captain Kirk would have given him a swift kick to the groin (conveniently located on his knees or something), grabbed his ID card and communicator, swiped his rather sleek-looking ship and done a fly-by of Shadow City to make sure they were all dead!
Lorien single-handedly reduced the ominous schemes and power-plays of ancient beings, to a petty squabble and told them to go to leave the younger races alone, or he'd send them to bed with no supper. He was the Deus Ex Machina brought in when JMS didn't quite know how to get rid of the all-powerful super-beings he had so successfully and intricately established. The series went downhill ever since he arrived - apart from the great pew-pew in the Shadow War of course. After he bogged off to the rim it returned to its usual fantasticness with the Earth Civil War.
He was far worse than anything in season 5, the rubbish films, and Crusade!!! Not because he was all that bad or annoying (he was, but others were more so), but he gets the award of "worst character" because he destroyed the plot, and caused the Shadows/Vorlon problem to be ended in a totally stupid way.