I am a huge Simpsons fan, i'm watching an episode from season 7 right now. They were awesome from season 1-10, 11 is kinda boring, 12 has a few good episodes and the rest is pretty much crap. At least it's better than family guy. It's all crap. South park is very fun, if i didn't grow up with the simpsons it may even be my favourite. So, simpsons > south park > family guy
Mid-series Simpsons and mid-series South Park are equally good. I would prefer to watch South Park, if only because I have seen the old Simpsons countless times.
Family guy is funny if you're smoked your mind out, but since I haven't done that in many moons it is unbearable to watch. The show can't keep a thought going for more than a few seconds.
Simpsons seasons 4-10 is timeless laughs.
I happen to think futurama trumps them all.
South park is great no matter which season.
Family guy is hit and miss. Sometimes good sometimes terrible.