I love all three. But if I have to pick one, it would be Futurama. I own all the DVD's and have watched every episode multiple times. I don't know why, but it never gets old. I can't wait for it to be back on air.
FWIW, I also own all of the DVD's that have been released for The Simpsons and Family Guy as well. But Futurama is what I watch most often.
Also, I've never been a fan of Southpark, the shrieking voices have always irritated me beyond belief.