South Park is one of those rare shows that, in my opinion, is getting better all the time. The first season was funny, made me laugh out loud which is rare, but now it's clever and makes me laugh every time I re-watch an episode. And that they can dish out brilliant stories that are relevant to current events in such quick time, just blows my mind.
As for Family Guy, even Seth Macfarlane thought it should have ended several seasons ago. I still think it's funny but it's lost it in recent seasons.
Also, American Dad > Family Guy