Porridge is still my most watched... I can flick over, see any part of any episode, keep watching and love it..!
Fools & Horses, as Gnash said, faltered in later years. Go back, watch the first couple of series, you'll see the difference. For me the turning point was Rodney's marriage.
Rising Damp was quality as was The Good Life.
Two others not mentioned, (for those of us old enough!) are George & Mildred (Youtha Joyce, genius!) and Mind Your Language... Remember them..??