In my opinion, Family Guy is definitely the best. I loved The Simpsons when I was younger, a few years ago, when the show was still funny and brilliant. I don't laugh or even smile anonyme when I watch a recent Simpsons episode. So I stopped watching the show at the end of season 17 or 18, I can't remember.
However, I started watching Family Guy when it made its comeback on FOX, and I quickly watched the full fist three seasons I've missed, and now that the show is in its sixth season, I can honestly say that it's really funny, entertaining and controversive. I really enjoy it! I also believe that the show is made for adults while The Simpsons is more of a TV-PG rated show, and we have to take that into account.