No, I haven't seen Avatar, but I have seen four of the movies nominated this year for Best Picture: Up, Up in the Air, Inglourious Basterds, and An Education. (By the way, thanks to a friend in NY, who is a Screen Actors Guild member and who sent me screener dvds of all of them!) They're all good in their own ways, but I want to be a heretic here and say that this year, the award should go to the movie with the least humans.
Up is the single best film on the list, consistently delivering everything it promises. Even more importantly, it's about dogs and friendship and trust.
Here's an article written by Glenn Close, who interviews Bob Peterson at Pixar -- one of the primary guys responsible for Up. He loves dogs, rescues them, writes about them, and seems like a hell of a guy.