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The first five seasons of Family Guy had charm and SOUL because they bothered to actually think. Yes, I'm even including the first two post cancellation seasons.
Right after the third one aired, season 6 (excluding the holdovers from the fifth season) was where things got really bad. Writing, the fucking animation, etc., you name it. They just stopped caring about making a good product. Literally look at Moving Out (Brian's Song) and watch as Marge comes along and gets raped by Quagmire. I mean, seriously, that's a very big downfall in quality, not even the previous season would do such a thing.
Fifth season was the last time where Family Guy was really good.
Anyways, early American Dad sucked because while they tried to think, all they could think about is George Bush. They found a bit more to tread by the second season though.