I have the "The Wire" box-set on the shelf, but no, I haven't seen it yet - I'm sure it would be up there as well, given that I'm a big fan of everything David Simon has his hands in...

Firefly is Joss Whedon's epic Sci-Fi western, that got killed after one season due to Fox... (That's a long story... google it ;-) ) The reason why it is so good is that Whedon's trademark dialog skills is shown to perfection - Its like Aron Sorkin, but funny, like Shakespeare but without the awkward poetry. Also the casting and the chemistry between the actors is just awesome and the world building that is so effortlessly established is amazing.