Family Guy used to be funny, and I have a very sincere love of everything they did from about the middle of season one through about the middle of season four, but it's gone so far downhill that it's nearly unwatchable. None of the characters are particularly likeable anymore, there's an absurd amount of focus on shock solely for the sake of shock, and the elements of basic storytelling -- which admittedly were not strong to begin with -- are nearly nonexistent. I wouldn't mind see it being cancelled again, but I know full well it won't be as long as it's a cash cow.
South Park is amazing to me as it's been consistently watchable for such a long time now. Sometimes I feel like it's running in place, but then they surprise me with something like the "you're not my buddy, guy" episode and I remember why I've been watching it all this time.
Futurama, as a work of creativity, is by leaps and bounds the superior show. It doesn't condescend to its viewers, it's eminently rewatchable, and it's breathtakingly clever and original. The movies weren't fantastic, and I'm wary of what the new seasons will turn into, but the storytelling of the first run was astounding. To wit: in a later episode in the series, Fry finds out from the sentient brains that Nibbler was responsible for his being frozen and sent into the past; that was the only way he'd end up without the delta brain wave and be around to thwart the brain's malicious activities. So Fry is beamed back in time to the night when he was frozen to confront Nibbler, who is under the table to make sure that Fry falls backwards into the cryonic tube. If you go back and watch the very first episode, for a brief second as Fry's chair tips over, YOU CAN SEE THE SHADOWS OF NIBBLER AND FRY FROM THE FUTURE UNDER THE DESK. Wow.