It depends if you're talking about the newer simpsons or the older simpsons.
I don't like Family Guy not because I don't find it funny, but I don't like it's style. It's just a bunch of random, pop culture references put together ina 22 minute episode that's sustained by a weak plot. Sure, the pop culture references can be funny, but I prefer Robot Chicken since it's the same thing except they have the sense to scrap the plot altogether.
As for South Park, well, I don't watch it so I can't judge it.