Family Guy, I admit, makes me laugh but at the end of the day, all of it's jokes are interchangeable. If I remember a certain funny moment, I honest to God, can't remember which episode it comes from. That's not good. I actually found recent episodes of "American Dad" to be better than Family Guy because of this.
As for the Simpsons, I guess it's because I was there for the beginning (it was my favorite part of the Tracy Ulman Show) but I still watch it. I still love it. I know the shock value of hearing a cartoon character say "ASS" or "Damn" isn't there anymore, but even when the jokes fail, the story is (in my opinion) still fun.
Simpsons for the win.
-----But Futurama is MORE WIN-ER!