From my understanding of Family Guy, for every 5 stupid episodes there is 1 hilarious one.
Seinfeld is for a more intelligent audience, so I get how you wouldn't find it good.
Simpsons=life.
South Park has some funny stuff, but is mostly just obscene or political references. I don't really watch it enough to have much of an opinion, but what I've seen can't touch Simpsons or Seinfeld.