For 2d, I really like using The Art of Flight. Gorgeous looking snowboard film that JVC was using recently to show off their projectors if that tells you anything. Pure demo material for A/V (video more than audio IMO, but the audio is fantastic as well). Avatar is another safe bet for either 2d or 3d.
For strictly 3d, Sammy's Adventure if you can get a hold of it is one of the best. Madagascar 3 is another fantastic 3d disc. Hugo is fantastic as well. Under the Sea is another great 3d must own disc IMO.
Lots of good choices for both 2d and 3d, but all those immediately come to mind.