Most people are just naming intentionally unlikeable characters, but if you wanna talk about weakly constructed ones, look no further than the female population of the show. I love The Wire dearly, but it is ultimately just not that interested in women. The big exception for most of the show's run was Kima, but she's really been pushed to the background lately (and the attempts to explore her personal life always struck me as awkward and half-assed). Aside from her, the female characters are all pretty marginal.
And yes, Valcheck is awesome. Thanks for reminding me about that "hairy-ass fuck" line. Gold!