I feel outnumbered here, but I have to say The Simpsons. I love all three programs and I watch them, but The Simpsons has been around 20 years and is still good. The Simpsons was the longest animated program on TV, then it was the longest sitcom, and I believe now it is the longest series bar none. That's quite an honor if you think about it.
Family Guy I'd put second. It's more avant-garde, and a lot of people just don't get it. It has wilder jokes and situations but the characters and plots are just not as good as The Simpsons.
Futurama I also love but it only took them 3 years to run out of ideas for strange planets to visit and strange robots for Bender to fall in love with.
But as I say, I TIVO them all, except I've seen all the Futuramas three or four times.