A most unusual post from a guy that joined in 2006 and has only 177 posts, after all it is a sitcom from 20 years ago. I'm a big Seinfeld fan, definitely in my top 5 for sitcoms.
I'd have to give #1 to All in the family . The main reason being it was such a revolutionary show when it came around about 1971 or so.so politcal and different from the popular shows of the late 60's stuff like Gilligan's Island, Bewitched, Andy Griffith, the addams family were popular, AF just so different.
Have to toss in MASH, Cheers,and a few others into the mix King of Queens is one that few would rate that highly but I like it, any movie with Kevin James sucks but KQ is very good.