I have to go with All in the Family. I've seen every episodes maybe three teams each. A lot of people wonder why I like it so much, especially at my age. Well it's because of the great storylines in every episode. I also love Archie Bunker the things that comes out of his mouth are priceless. His facial reactions are hilarious as well.
I'm partial to any old british comedy sitcoms. Blackadder and Fawlty Towers are my favourites I think, but there are lots more. The only more recent sitcom I really watch is Everybody loves Raymond, because it was always on TV in the morning before I went to school, and I still catch it on every now and then. Oh King of Queens, too.
I could never pick just one because I have too many favorites, including:
1950s: Life of Riley, I Love Lucy, Honeymooners
1960s: Beverly Hillbillies, Munsters, Get Smart
1970s: Happy Days, All in the Family, One Day at a Time
1980s: Cosby Show, ALF, Simpsons
1990s: Everybody Loves Raymond, Frasier, King of Queens
2000s: Big Bang Theory, Gary Unmarried, Two and a Half Men