Has to be South Park for me, although it's pretty close. I've been watching a Family Guy marathon all day, and it's been pretty funny. Still, South Park is guaranteed to get a lot of laughs from me, so it's my fav of the two.
Futurama
South Park
Family Guy
American Dad
Simpsons
Cleveland Show
There were about 3 years where Family Guy was clearly the top show. I still watch it every night, but since American Dad and Cleveland show have taken off, it seems like they really phone in Family Guy these days.