I know some of you are questioning the inclusion of animated shows, but King of the Hill is definitely one of the best. I would take it over the golden years of the Simpsons (which I also love).
I never appreciated it when it first aired. Was expecting a Beavis and Butthead spinoff, something more crude and offensive along the lines of other animated sitcoms that came before and after (Simpsons, South Park, Family Guy, etc.). Plus I was also a teen when it first came out, so those more in your face shows were definitely what I wanted. So watching King of the Hill when it first came out, expecting something much different than what it was, I just didn't connect with it.
It wasn't till near the end of its run, when it was the only thing airing on a plane, that I watched it and realized how hilarious it was. The humor is just so dry, not like anything else (and definitely not like any other animated show), so if you're not paying attention you can easily miss half the jokes.
And due to it's dry style, still holds up incredibly well. I'm constantly watching re-runs of King of the Hill, more than anything else. The re-watches hold up better than the majority of other shows.
King of the Hill might be #1 for me, animated or otherwise.
Cosby Show is probably top 5. When it first aired, it's just what me and my family watched on Thursdays at 8pm. But watching re-runs as an adult, also still hilarious, and so many little things about the humor I didn't appreciate as a kid when I just saw it as a family sitcom.