While it's difficult to choose a favorite scene, one of mine is the part where signal fire after signal fire roars to life as Minas Tirith calls to Rohan for aid (thanks to Pippin), and the music soars with hope. Théoden makes his decision:
Aragorn: The beacons of Minas Tirith! The beacons are lit! Gondor calls for aid!
Théoden: And Rohan will answer! Muster the Rohirrim! Assemble the men at Dunharrow, as many men as can be found. You have two days. On the third, we ride for Gondor… and war.
That bit always gets me.
Mind, this part in Fellowship always gets me, too (Sean Bean was terrific as Boromir!).
Boromir: Give them a moment, for pity's sake!
Aragorn: By nightfall these hills will be swarming with orcs! We must reach the woods of Lothlórien. Come, Boromir, Legolas, Gimli, get them up. On your feet, Sam. Frodo? Frodo!
The performances by Sean Astin and particularly Elijah Wood -- his eyes, the loss of innocence in his eyes ... he is completely bereft -- never fail to blow me away.