Oh boy, I'm going to get some hate here.
Family Guy is my favorite. its just so unabashedly evil, and while South Parks evilness seems to come from just shock value, Family Guy manages to make more clear that they have planted tongue firmly in cheek. Its just so Wry, and really makes fun of the darkest pieces of peoples minds. Also, as a Robot Chicken fan, I gotta love the gags, and the pop references are masterful.
Simpson's was outdated for me for a while. It started shakily for a season or so, and turned into something top notch, if also the only game in town for adult appealing but somewhat kid friendly comedy animation. It became a cultural institution. With time, it fell toward the surreal (Pretty close to the time Family Guy rolled around), which was funny at first, but eventually became grating. It felt like a watered down Family Guy. It just had no heart, and a little heart to temper the craziness is what made Simpsos. I recently watched the most recent season, and though occasionally they seem like there trying too hard to be relevent, they are still funny, and that little seed of heart is back. So Simpsons jumps to second.
Futurama is not bad by any stretch of the imagination. I watch it regularly. But it still feels like a Simpsons spin off that's trying just a bit too hard. Feels a little forced to me. It's third place, but remember that a bronze medalist in Swimming is still a DAMN fast swimmer.
Basically, I like em all.