Not as funny as family guy, and defenitely not as funny as south park. For one thing, there are no real cutaways in american dad - there is randomality and discussions that are outside of the context of a standard conversation, but not like in family guy. In family guy, at least in the first seasons, every random remark was accompanied by a cutaway, which made you think that maybe, just maybe, the writers of family guy are manitees (as south park suggested in the episode cartoon wars).
In conclusion: family guy is hilarious, south park is just as funny, american dad does not even reach their heels.