Lots of great series have been mentioned, but The West Wing would have to be may all time fave. The quality of the writing was beyond compare. Outstanding TV, and there isn't that much of it around.
Some other faves:
My So-Called Life (revisited this recently and it holds up pretty well even though I am no longer a teenage girl!)
The Vicar of Dibley
Absolutely Fabulous
Homeland (hopefully the second series is as good)
Queer As Folk (both UK and US)
House (I have a weird thing for Hugh Laurie, there I said it
)
ER
Extras
Blackadder
Who Do You Think You Are? (UK, Aus or US versions)
Anything on my television that involves Stephen Fry.
Some Aussie stuff:
Love My Way (all time favourite Australian show, series 1, episode 8 is devastating and one of the best pieces of Australian television I've seen)
The Late Show
The Games (mockumentary about organising Sydney Olympics)
Kath and Kim
Prisoner (classic set in a women's prison... with warders by the name of Vinegar Tits and The Freak. So bad it's good!)
The following three shows are all by Chris Lillee. I honestly don't know how they'd be viewed by a non-Australian audience. Polarised a few people here but I loved them.
We Can Be Heroes
Summer Heights High
Angry Boys