I'm gonna have to make two lists, one drama, one sitcoms. They're just too different to compete.
Drama:
The Wire
Deadwood
Cracker (UK)
Homicide: Life on the Street
The Shield (Only recently started watching this, up to the third series. It's kind of like The Wire-Lite, similar in some ways but just nowhere near as sophisticated, resonant or well written. But it is good to see some true moral ambiguity from a US TV show)
Sitcoms:
Fawlty Towers
Peep Show
Seinfeld
League of Gentlemen
Red Dwarf
Drama:
The Wire
Deadwood
Cracker (UK)
Homicide: Life on the Street
The Shield (Only recently started watching this, up to the third series. It's kind of like The Wire-Lite, similar in some ways but just nowhere near as sophisticated, resonant or well written. But it is good to see some true moral ambiguity from a US TV show)
Sitcoms:
Fawlty Towers
Peep Show
Seinfeld
League of Gentlemen
Red Dwarf