Family Guy is always just so hit and miss. When it hits it's bloody fantastic, but there are far too many cringe-worthy parts for my liking. Not even fit to be mentioned in the same breath as the other two (although much better than the likes of American Dad and King of the Hill).
It's hard to split the other two. South Park can be a bit hit and miss as well, but have a far, far better success rate. Whereas Simpsons is (or was) more consistent, but probably had less 'laughing so hard I've got tears coming out of my eyes' moments. Futurama is close behind those two as well.