Gotta break this down by decade. The main criteria I use is the timeless quality of the show, how well it holds up over time.
1950s -- I Love Lucy. Set the standard for sitcoms.
1960s -- Andy Griffith. Could be screwball comedy (i.e. Ernest T. Bass), could be very touching (i.e., Opie and the baby birds).
1970s -- MASH. Excellent ensemble that spent 11 years in a series set during a war that lasted 3 years.
1980s -- Roseanne. Okay, maybe it's because I'm watching this a lot on Nick @ Nite lately, but this series still makes me laugh out loud ("Becky cut the cheese" -- man, Darlene has great comic timing for a kid!!!)
1990s -- Seinfeld. Eccentricity at its funniest.
2000s -- too soon to tell.