The original seasons of Family Guy were actually very good for when they were released... but the fact that it became so successful and mainstream makes them seem played out, lame, predictable now.
The thing is, originally, Family Guy was meant to be a subversion of typical cartoon family tropes, taking twists and turns with stories that were, at the time, completely unexpected. This doesn't really work when the twists become the mainstream.
Unlike South Park which was just toilet humor with awful production values across the board(Nothing has changed on that front), Family Guy, on release, was a labor of love of some individuals who clearly put in a great deal of effort into their work.
For the record;
I don't watch, or enjoy, Family Guy, but I would enjoy South Park(I.e. garbage) even less.