They've all got their positives and negatives. Of the lot, Simpsons is probably the most consistent with what they do, though many will argue that the qualities gone downhill since season 10ish. Humour is very sitcom based, where comedy is derrived from situations and characters.
Family Guy is very hit and miss. It's one of those shows where the greater your understanding of pop culture, the greater you chances are of getting the joke. Has a basic grasp of surrealism with its humour, and non-sequitors abound.
South Park tends to be more socially and politically motivated, and more extreme in terms of language, violence, and themes. It terms of comedy, it relies heavily on pop culture parody and satire, and well as juxtaposition.