I take it from the replies that this thread is limited to US drama series?
Not AFAIK, but it's interesting that most of the suggestions are form the USA. I have to say I agree: the standard of UK drama is dire ATM, with only Rome worth watching that I remember.
I'm inclined to agree that The West Wing is the best. The Wire is good, but a little traditional. House was good for two series, then ran out of ideas. Lost is complete nonsense. Dexter is good (I've only seen the two terrestrial series). NCIS is fun, but hardly classic TV, any more than CSI is. Although the first three CSI (Vegas) series probably deserve to be in. If we're allowing WEst Wing (started late 1999 in the US at least) then I'll add Law and Order: Special Victims Unit.
M