So, in no particular order:-

As If (UK) - forerunner of Skins in many ways (teen ensemble piece, with variable viewpoints). Sort of obscure and never released on DVD (Grrr).

The West Wing - everything I know about US politics I know from this show. Utterly, utterly wonderful.

Studio 60 on the Sunset Strip - Aaron Sorkin can do no wrong, all I'm saying.

Skins - I'm so emotionally invested in this show it's not even funny.

Battle Star Galactica - best sci-fi ever mande? Y/Y

Farscape - more science-fantasy than sci-fi maybe, but still a glory to behold.

Life on Mars (UK) - John Sim = ♥ and Gene Hunt is just glorious.

Spaced (UK) - geek heaven. 14 episodes of comedy gold.

Buffy - my first true love. I have never, and probably will never, loved a show as hard as I love this.

Firefly - cut off before its time. A masterpiece.

Dr Who - needs no comment.

Sugar Rush - funny, poignant and totally wonderful.

(The X Files stopped being worth watching long before 2000, but the early seasons were pure brilliance).

There's probably more, but these are all that spring to mind right now :)