Early Simpsons episodes are untouchable in terms of writing. From series 1-7ish.
South Park has got better with age. The writing style has improved and the episodes they 'get right' are unbelievable.
Family Guy was good at a time. I don't think it could sustain it though. Some jokes are too long, too American, not funny and seem to be Seth McFarlane's opinion on things more than anything.
American Dad is not funny. It's really not funny.
So there. That order, for those reasons.
EDIT: Had to go on YouTube didn't I!
[youtube]http://www.youtube.com/watch?v=6kW4uYInAFE[/youtube]