I get angry when I watch new episodes of the Simpsons, since their earlier seasons beat out any other show, in my opinion. Unfortunetely for several years now it has been on a steady decline. Seasons 4-6 of the Simpsons will beat anything South Park or Family Guy will ever do
I think Family Guy is the worst out of the three by far, and though they have had a few pretty funny episodes, most of the jokes are just nonsensical flashbacks.
However right now, South Park takes the cake, they seem to be coming up with the freshest, funniest material out of any of the three right now, and haven't shown much signs of slowing down yet. Some of the characters have developed really well over the years as well.