Mystery Science Theater 3000 has some episodes where the movie is too bad to be enjoyed, but more often than not it's one of the funniest things ever. It's impossible not to have this in the top 5.
Mr. Show is sickeningly overrated, most sketch comedies are more miss than hit, but this one is mind boggling. Kids in the Hall is better, and a few early Python stuff, as well.
Most of the popular comedies like Friends and Seinfeld are even worse, I don't get how people can still laugh at that typical sitcom crap. A few traditional sitcoms like I Love Lucy and Three's Company are good shows, though.
There are shows like The Tick, Andy Richter, The Critic, and more which could have been up for contention had they lasted longer. Family Guy was only 3 seasons but I think it can still be one of the funniest ever. And of course Simpsons is there.