The Wire is absolutely one of the most underrated shows on television. But having said that, I wouldn't say it's one the "best" out there, at least not compared with the greats (won't bother naming 'em).
The show is much deeper than it appears. Many little things going on, many little subplots. All, in some way, connected... Not to mention, viewing the perspective of both the streets and the police/narco's is very cool. Can't forget all the politics as well, with Carcetti (sp?) trying to make mayor, etc.
Sweet show. HBO is one the most innovative channels when is comes to original programming. Can even go back to some of their first shows, like Dream On
or The Larry Sanders Show
(maybe some of y'all were a little too young, I was pretty young at the time too).