My top three:
1.) Twin Peaks (Hence the avatar). I never got in to it at the time, but Amazon kept recommending it to me, bought the first series for fifteen quid, and fell in love with it. It manages to be funny, scary, dark and uplifting. It goes off the boil a bit halfway through the second series, but the ending is something else.
2.) The Wire. Hugely realistic (or at least I think it is, never having been to the mean streets of Baltimore). It takes a little while to get in to, as the story builds up over the first few episodes of a series, rather than giving huge and ridiculous cliffhangers at the end of every episode, but the payoff is fantastic when the action comes. The characters are brilliant, and there's no sense of 'the good guys' vs. 'the bad guys'; every character has his flaws and his redeeming features. My only criticism is that after watching it, every other TV programme seems fake.
3.) The Sopranos. Seriously, between this and the Wire, I want to live in the US just so I can get HBO and catch this stuff earlier. Again, brilliant characters, and a lot quicker-paced than the Wire, but there are brilliant moments throughout, including the final scene, which is probably my favourite TV scene ever.
I also enjoy(ed) Lost, Battlestar Galactica, Rome, Jam (seriously twisted, that one), Family Guy, American Dad...