First of all, I've not seen Avatar nor the Hurt Locker. But that's not important for illustrating my point.. Critically speaking, 'best' often relates not to the impact a film has on society, but on how well the movie succeeds in advancing the technical possibilities of film making-- technical in this case referring to matters related to the craft of filmmaking. This is one of the major factors going into the selecting of movies for the Criterion Collection, which are not known to be 'enjoyable' so much as they are technically interesting.

Given this definition of 'best', I think the Academy is always pulled in that direction or in the direction of the mega-blockbusters. If they choose the latter every year, they risk being sell-outs. If they prefer the former, they'll lose interest in the mainstream.

 There really wasn't a good option in the middle. Basterds was not mainstream due to the excessive violence and IMO, frustratingly rife with technical problems. I can't think of another one that might contend.