I haven't watched The Wire yet, but Six Feet Under is the best of the series I've seen. And as this guy says- the final sequence of SFU is stunning. Sometimes I go back and just watch the ending on YouTube. Also Buffy the Vampire Slayer should be on this list.
X-Files Star Trek Star Trek TNG I guess none of you people ever liked Quantum Leap? (I never liked it, but I remember a LOT of people going crazy over it back in the day).
It is damn near impossible for me to pick between Rome and LOST. I finally picked Rome because I figured it would have less votes.
My favorite is Firefly. Its hard to call it a series since it had so few episodes and was not allowed to end. However every show was excellent.