As people have mentioned elsewhere, Pixar on Blu-ray is fantastic. Ratatouille and Cars are both a lot of fun. I've enjoyed audio and video on other Blu-ray discs, but I'm still digesting which ones I like the most. Most of them are better than any DVD for showing off a system, though. I haven't tried to demo my system for a while, but I've used Apocalypto on Blu-ray, and I have to say I enjoyed No Country For Old Men on Blu-ray, too. Some of the gunshots are very satisfying. For more LFE, try X-Men: The Last Stand.
Man, I still have Blu-rays I have yet to watch.
