I still find myself using the second scene in Gladiator where the Romans are annihalating the German horde for demoing. This is probably the best scene I have come across from a decent movie, that has surround sound along with other elements. U571 has some awesome scenes for the bass which are better, but nothing to go with it. Master and Commander is very close in some of its scenes.

I love that and the Battle for Carthage, the first of the Gladiator fight scenes in Rome.

I was not very impressed with the surround effects in Gladiator. Do not get me wrong. It was an excellent movie. Just does not have the impact for a demo. Surround effects are much better in movies like:

Lord of the Rings (Extended Versions) DTS 6.1 tracks - Any of the many battles
The Incredibles - Chase scenes on the island
Pirates of the Carribean - Fight scene with the black pearl
Bad Boys II (Superbits DTS track) - Car Chase
The Grudge - All scenes with little boy running around
Harry Potter Movies - Any Quiditch scene
Finding Nemo (Full Screen Version) - THX Intro with flying robot