Saving Private Ryan is a good demo, albeit a bit "long in the tooth." I think it's been eclipsed by some better discs. However, the movie itself is terrific- so many "demo discs" are worthless for actually watching.
I'll also say the demo thing has sort of played itself out for me. I've rattled all the windows with every F/X spectular out there, now I'm more into subtlety. I second Master and Commander, much more complexity and detail than most. There's bombast in the battle scenes where the cannonballs crash thru the planks and you can hear the splinters rain around you, but you get great sounds of the sails creaking, etc too. Likewise, The Red Violin is a great demo due to the skillfully played and recorded music (mostly by Joshua Bell). It helps that both of these are really fine movies.
If an F/X bonanza is what you want, Final Fantasy is very impressive too. Add to that list Titan AE for some incredible LFE (especially in the Ice Field scenes and the beginning where Earth is blown to bits). And of course the DVD set of the original Star Wars trilogy.
