Chapter 2 of Ratatouille (Blu-Ray). Gorgeous picture, stunning use of the LFE channel (the thunder rumbling in the background, the lightning strike, and the shotgun blasts). It's family friendly enough to demo with young kids, and everyone who sees it thinks "wow, I have got to get a setup like this!"
The opening scene of Fellowship of the Ring is good too (the Sauron fight). Good use of surrounds during the battle, and the LFE effects are intense when Sauron starts swinging his axe around. Anytime the ring is shown on screen, the HZ drops into the teens, which is great for rattling people's fillings out. I actually managed to shake dishes off a shelf in a kitchen upstairs when Sauron exploded.
If I'm demoing for family, I use Planet Earth (blu-ray). The visuals are stunning, the cinematography is gorgeous, and I know how much they love animals and nature shows. Watching a great white leap out of the water to grab a seal in super slow motion is just awesome.