[Gwathagor]

Charge of the Rohirrim at the Battle of the Pelennor!

OR

The lighting of the Beacons! (though there were too many)

OR

The part where Frodo and Sam are lying on the side of Mount Doom (before they have destroyed the Ring) and the flaming rocks are falling through the sky and James Galway is doing his flute thing. I like that part.