The charge of the Rohirrim at the Battle of Pelennor Fields. Jackson managed to one of the few directors ever to convoy the sheer shock of a disciplined force of trained horsemen hitting a line at full charge.

Pretty much every scene with Sam. Frodo really doesn't develop as a character, while Sam goes from a rather shy gardener to the real hero of the piece (IMHO.)

I'l amost ashamed to say I've never finished reading the books. Tolkien's writing style leaves me utterly cold. I've tried several times, and never gotten much farther than the first few chapters of The Two Towers before giving up. The movies, however, were epic.