I've been planning to do a Top 5 70's sitcoms for a while, but in compiling it I realised one of my choices (Terry & June) was really an 80's series. It's kind of snowballed from there, so I now have Top fives for the 80's, 90's and 00's as well. If this isn't you're bag you might want to come back in 5 weeks or so when it's all over! Might even manage a Top 5 US Sitcoms if you're lucky. The seventies seems a great place to start though. For me (at an impressionable age) it was prime time for British situation comedy.
1. Dad's Army (1968-1977)
9 Series written by Jimmy Perry & David Croft
Probably my all time favourite sitcom. So many fantastic characters and some great lines throughout it's long life. The "Don't tell him Pike!" sequence still makes me laugh today.
2. Porridge (1973-1977)
3 Series written by Dick Clement & Ian La Frenais
Consistently brilliant from start to finish. Ronnie Barker is superb and his on screen chemistry with Richard Beckinsale is wonderful to see.
3. Are You Being Served? (1972-1985)
10 Series written by Jeremy Lloyd & David Croft
Classic stuff during the 70's period. Probably ran for 5 years to long and the quality tailed off dramatically as the writers increased its "sauciness" at the expense of actual humour. It also lost a lot when Trevor Bannister (Mr Eve) left in 1979 and the original Mr Grace (Harold Bennett) died in 1981.
4. Citizen Smith (1977-1980)
4 Series written by John Sullivan
Possibly my memories of this are swayed by Wolfie being a Fulham supporter, but this was revolutionary stuff at the tail end of the 70's.
5. Fawlty Towers (1975-1979)
2 Series written by John Cleese & Connie Booth
Not sure I like this as much now as I did back then. Farce is never the most subtle vehicle for comedy but the cast play it well and with only 12 episodes ever made it disappeared before it had a chance to really grate.