Many times magazines and online articles mention different movies that would be good demo material. But, when I've used such things when people come over to the house and want to see and hear what my system can do, it's usually pretty mixed results.
Here's what I have noticed. If there are older folks, like my parents or in-laws that want a demo, they could care less about some big action/adventure/sci-fi/ fantasy movie, which is where all of the good demo scenes come from. But, if you get a concert Blu-ray disc, as long as it is an artist that they like, then they are much more likely to enjoy it. The same goes for most women as well. Most don't really get excited about explosions, guns and car chases.
I find concert Blu-rays almost better than being there. Since there isn't a big selection that you can rent, I've been just purchasing them and, like I said, when someone comes over and wants to check out my system they make a very nice demo.
There are a few movies that should work universally well for almost anyone that wants a demo: Master and Commander chapter 4, Jurassic Park when the T-Rex is creeps up on the car in the dark (but there isn't a Blu-ray out yet).