Monty Python on TV is in a league of its own, really. Many sketches are still very funny, even though fans already know the punch-lines (or can recite whole texts from memory!). The intellectual and the farcical has never been combined so well since.
I'm not a big fan of sitcoms (especially recent British stuff)... but I liked
Frasier,
M*A*S*H, and off-beat shows like
Monk.
Yes,
HIGNFY, was always a laugh with Merton and Hislop... but it lacked proper focus after they sacked anchorman Angus.
TV stuff I particularly enjoyed was the Comic Strip presents... series. Sometimes brilliant satire!
Strike! was a classic... (Who can forget Peter Richardson playing Al Pacino as Arthur Scargill?)
GLC was another work of rare comedy genius... (with Robbie Coltrane playing Charles Bronson as Ken Livingstone... and there was theme song by Kate Bush).