Agree with everyone that the Simpsons has gotten somewhat monotonous. Of the two, I think the Simpsons has more "classic" episodes, (but then it does have some 16-seasons on Family Guy), but I think Family Guy still has a lot of life left in it to keep going, should it be brought back on air. Hard to cast my vote either way though, since I think that The Simpsons in its day is far superior to Family Guy, but equally with Simpsons' on going monotomy I've recently prefered to watch Family Guy.
Do I really have to choose?...Can't I just be greedy and like them both?

Haven't seen American Dad myself, but several friends of mine (big fans of The Simpsons and Family Guy) have highly recommended it...

To throw stones in though, personally, if it was an option, I'd go for Futurama as one, if not the, all time favorite animated series of mine. That, or the original Teenage Mutant Ninja Turtles - now that was an animation series!