I honestly do not think they should be compared.
They are both brilliant in their own ways!
But if we must compare them, then I will say The Simpsons! I hope people arent comparing todays family guy to older Simpsons episodes. There is no contest in humour there, however todays Simpsons I find funnier than Family Guy. Plus I grew up on The Simpsons!
The stories are better on The Simpsons IMO as well, especially the older ones, mid 90's.
South Park is also brilliant, and I would put it ahead of Family Guy but slightly behind The Simpsons in my top 3! South Park is different from both though, and again I would not compare them.
I love South Park in the way that, a lot of the episodes start of with something so small and basic, and ends up with something outrageous.
For example, The episode with the whale (cant remember its name) at the start of the episode the people working at the sea world type place are messing with Kyle by speaking over the tannoy pretending the whale is talking to Kyle, and saying that it lives on the moon and wants to get back their. Thats something really small.
However, by the end of the episode they are flying a whale to the moon with a rocket lmao Love it!
Theres also the one where Mr Garrison gets a sex change, and Kyle wants to be a tall black kid and ends up actually being one, and his dad also gets turned into a Dolphin. Youve gotta love South Park.
However I prefer the episodes without as much dirty stuff in lol I show that to my mum and she wouldnt want to watch it, however she found the episode where they get The Dog Whisperer to train Cartman hilarious.
Cartman is the obvious favourite, and I also love Butters.