The Wire is, in my opinion, the best show in the history of television (or at least of what I've seen). The Wire seems to have received a lot of critical acclaim and a cult following, especially after its end. Too bad it never got the popularity it deserved when comparing it to The Sopranos. Although I like The Sopranos, I think The Wire brought a lot more to the table in terms of writing, gritty realism, and pure entertainment.
One thing that can be both a quality and a problem, is that The Wire is very demanding. A lot of what happens is implicit or not directly seen on camera. Also, episodes are not as self-contained as, for instance Mad Men, where you can watch one stand-alone, without needing too much background on who's who, and what happened before.