[Blu Falcon]

These are the 3 movies that always generates "oohs and aahs" when I'm showing off demo material to friends...

Cars (for pure perfection)
Sunshine (to demonstrate how a dark film can be mind blowing in high def)
Galapogos (the underwater scenes - need I say more?)

I'd also add The Rock or Con Air to demonstrate how older films should look in high def.

EDIT: Speaking of older films, Blade Runner and Close Encounters... would make good demo material as well. Most of my titles are top tier PQ wise, so I can put on almost any movie and blow people's minds.