First thing that springs to mind is the Christmas Eve adaptations of MR James stories. There were two made in the 1970s that were masterpieces (Whistle and I'll Come to You was one, I think).
Mark Gatiss' documentary on M R James is also a forgotten masterpiece
I Claudius (1970s)
Colditz (1970s)
Brideshead Revisited (1980s)
Cranford (2007, with Judi Dench)
Bleak House (2005, with Charles Dance – easily the best ever adaptation of a Dickens novel)
Andrew Graham Dixon: Secret Lives of the Artists
Matthew Collings: Hello Culture (he also did a brilliant series on modern art)
The Office (the original British series, not the American re-make)
Kenneth Clark: Civilization
Alan Partridge: The first series, when he had his chat show
Bruce Parry: Tribe
Fawlty Towers
Blackadder: Final series (set in WW1)
Parade's End (Benedickt Cumberbatch, script by Tom Stoppard)
Sherlock Holmes (starring Jeremy Brett)