I picked Futurama because it has the best characters in my opinion. Of course, it's hard to choose from such a great list.
South Park is often absolutely brilliant but for me is too hit and miss. Particularly in more recent seasons, there is usually one or two episodes that is just not funny. Other times, however, they hit the nail right on the head.
Family Guy is also hit and miss but they pack so many gags into one episode, it doesn't really matter. A lot of the humour is silly and immature but it's damn funny. I think some of the funniest moments I have ever seen have been on Family Guy.
And of course, who could forget the Simpsons? Remember that this show's been going for 20 years and it's still funny! Futurama and the Simpsons are obviously both created by Matt Groening and they're both brilliant but I'm a sucker for Sci-Fi so Futurama wins.
Adult Swim? Yeah, they've had some good stuff like Harvey Birdman or ATHF but they seem to be a bit lacking nowadays. Certainly worth an honorable mention.
In conclusion, they are all great in their own ways. Which is better? There's only one way to find out...