In no particular order:
Carol Burnett Show
MASH
Nature and
National Geographic
WKRP in Cincinnati - agree with a pp about the Turkey episode
Cheers
Friends
Star Trek - just because it started it all
Star Trek - the Next Generation - because of its superior dialogue and characterisation
Babylon 5 - I think it does stand the test of time, even with the outdated CGI, DS14 loves the show even with a steady diet of sci fi with superior effects. Story wins out every time; and the concept that the writer had the whole 5 years mapped out before they started filming is amazing.
Wonderful World of Disney
Barney Miller
Dr. Who - through the ages, but my fave is still Tom Baker
Columbo - not sure if it was the first, but it was probably one of the better detective shows that added humour and led to a whole genre. Please correct me if I'm wrong, I wasn't allowed to watch Columbo when it first came out and only saw episodes later. But I did watch all the other detective shows that came after like Rockford Files, Simon and Simon, Magnum, PI, etc.
I can't think of any more that haven't already been mentioned.
I have to say that I tend not to watch network tv too much anymore as I prefer to watch channels such as Discovery Civilization, History, NG, etc and because so many of the network shows are disappointing. I have to agree with Stinkerbell's Daddy though about Big Bang Theory, it's my favourite current sitcom.