A lot of shows that I thought were brilliant at the time don't stand the test of time, like the X Files or Blake's 7, so they're just nostalgic now (this mostly applies to cop shows and sci-fi which date quickly as society and technology changes).
I have to divide my choices into American/British because they're just so different, like The Office or Life on Mars UK versions versus the US ones. Even the same thing is... different. One just would never be made in the other, and vice versa:
American (mostly sci-fi/geek, what the Americans do best):
Battlestar Galactica
Stargate SG-1
Firefly
Veronica Mars
Toss up right now between Chuck, GR∑∑K or The Big Bang Theory.
British (all comedies, what the British do best):
Black Adder
Red Dwarf
Men Behaving Badly
One Foot in the Grave
Father Ted
Special mention - new shows -> Dollhouse (American) and Misfits (UK). Dollhouse got better as it went on, while Misfits was already verging on brilliant 'cult classic' after the first 10 minutes of the very first episode. See what the second seasons of both bring!