To clarify my prior post, Futurama is tops in my book, Simpsons and Family Guy tying for second.
I don't know what it is about South Park, but ever since I saw it for the first time, I've thought that the show is somehow beneath me. They've got talking poo for crying out loud. Overall I don't find it humorous at all, but I do believe that Matt Stone, and Trey Parker should have been raised better. The other shows listed occasionally have immature (IE toilet) humor, but overall, keep things on the up and up.
For the record, American Dad is also an awesome show. It is to Family Guy, what Futurama is to The Simpsons.