I really liked Superbad only for the awesome that is Michael Cera. Other than that, I'd say 40 Year Old Virgin is my favorite one. It had the most consistent laughs for me, Steve Carrell is just awesome, and you can't beat that supporting cast.
Knocked Up was really good, but as people have said before here, it's a little too mushy. Seth Rogen's good stuff though.
Haven't watched Freaks and Geeks, Undeclared, or Heavyweights yet.