All of these are great choices. These are my personal favorites (in no particular order):
-Master and Commander DTS: Simply an exquisite audio experience. Awesome sound design, tight but powerful use of subwoofer, constant activity in all channels ... this is the movie that audio engineers should study.
-Pixar Movies / Cars: I watched Cars last night and while it was a surprisngly weak movie for a Pixar film, I though that the AV presentation was reference quality.
-LOTR - Fellowship of the Ring DTS: This is my favorite mix of the extended DVDs. It just sounds much nicer and somehow feels less compressed and more directional. Its achilles heel, unfortunately, is an unwieldy and unnecessarily loud LFE
-Mr and Mrs Smith: I was watching this on HBOHD the other day and was surprised at how well made the audio presentation was. The spouse fight in the house and the car chase immediately come to mind.
-HBO Series: I have been watching Deadwood and Rome DVDs (thanks Netflix!) and in HD. These have some of the best sound designs on TV ... or in general! Music comes across wonderfully, dialogue is clear, and all of the small details (creaking leather, voices coming from the left, etc) come through in an effective manner.
I like the others that have already been mentioned, but these are my favs.