I find that the Discovery Channel's Planet Earth and Blue Planet DVDs or Blue-Rays just blow people away.
I first played them at a friends house at a party, and it was the first time everyone at the party actually crammed into the living room to watch the episodes and we ended up watching the whole series.
My other favorite is Lord of Rings even though not in Blue-ray format -- the movie still blows me away.
Iron Man is also awesome to watch
My new favorite after watching it during Thanksgiving is the movie Up! -- We watched at a family members house who has a 1080p projector and it was amazing!
Agree as well, Pixar animated movies, dark knight, iron man, I am Legend,I-Robot the list goes on and on. If you want to impress with Awesome surround sound try The Incredible Hulk 2 Bluray, The warehouse scene were the soldiers first meeting with hulk - (shooting scene) it will blow you away!
Dark Knight is great for showing off black levels and shadow detail. If you really want to pop their eyes out of the sockets pop in the Fifth Element. The amount of detail and the amazing color and cinematography make this a great demo disc.