Hard to choose one (as always seems to be the case). There are the obvious ones (Friends, The Sopranos, Doctor Who) but the three below are also worthy of mentions.
The Office (UK) - Still one of the best comedies the UK has produced. Gervais, Crook, Freeman etc. are brilliant in it.
Top Gear (UK) - Gone off the boil a bit recently but the series as a whole has provided superb entertainment. The banter between Clarkson, Hammond and May is great aswell.
Cracker (UK) - The king of crime drama's.