Its a tough one. I think Family Guy is probably the most consistent, and I've laughed harder at individual jokes in it that than I have in South Park. The Simpsons, whilst less consistent as a series has far more episodes which are funny and clever throughout. South Park is awesome for going full on when it picks a target, it can be clever with it too.

My vote goes to The Simpsons almost just out of loyalty. The other two are just shows, albeit courageous ones that deserve a lot of respect. The Simpsons is something a little more.