I voted for season 2.
Anyway, I think some episodes in season 4 is really lame, but some episodes are really good. I liked "Where's Gary?", "Patrick SmartPants", "That's No Lady", and "Bummer Vacation" especially. And I think the newer episodes are getting better than the beginning of season 4. =)
Oh, and one more thing: Why didn't season 1 get any votes? O_o Even though the animation isn't high, it's still a good season (my opinion)! Bye.