Irritating is pretty subjective, but weakest? McNulty, as a character, exists to highlight the criticism of the police force which is so central to the story. And he´s broken for it, just like Daniels is, which is one of the main criticisms of public institutions the show has. He´s also an anti-hero, the guy who sees how the system is rotten and won´t stand for it, but who´s a crappy father, an alcoholic, and a pain in the ass. I wouldn´t call him weak.