I couldn't stand Brother Mouzone. In a show as grounded in reality as The Wire there was room for one walking cartoon character (yeah, I know Omar was based on a real person, whatever) but throwing Mouzone in the mix was just overdoing it. At least he didn't overstay his welcome.
Lester Freamon is kind of a cartoon character too (I just started season 2, maybe that changes) but he's likeable enough. Only character that bothers me so far is McNulty.
I think people get bent out of shape over Omar/Mouzone being too unrealistic, but they're probably some of the things that kept the show from being just too fucking depressing to bear.