The Simpsons has delivered 25 seasons of (on average) 'good' TV. It has some absolute classics, especially seasons 3-8 but even now I'll see episodes from the last few years and they do push the envelope in an 'acceptable' way and I'll really enjoy them.
Now, I love Family Guy... but what The Simpsons has achieved is actually remarkable and historic.
South Park... I watched the World War Zimmerman episode last night and found myself getting really, really uncomfortable. I definitely think SP is more hit and miss than either of those two.
Neither the Simpsons nor Family Guy even touch South Park at it's peak from 1998-2004. SP was utterly hilarious from start to finish for several seasons.