The Battle of the Five Armies. It goes beyond epic into almost a surreal, dream-like realm of battle, which I kind of think is what a real battle would feel like. "This isn't over yet. We have to keep fighting. Just stay on your feet and keep fighting." Like with Helm's Deep, this battle leaves all the characters exhausted and spent, and I'm the same way. Unlike Helm's Deep, though, it's not kind of the same thing over and over -- there's more variety with all the different fronts. It feels more personal too because you have Thorin squaring off with Azog again, Legolas taking on Bolg again -- these are major showdowns we've been needing to see because the movies have built up to them for so long. So I find the whole battle very satisfying on a storytelling level too, because it brings so much completeness to the story.
Also, it's more heartwrenching than the other battles, for me, because characters I really care about die. Sure, major characters died in other battles, Theoden, for instance. Haldir wasn't a main character, but I was sad to see him fall. But never so many or with such emotional results. (Do I need to say SPOILER ALERT at this point? The DVD isn't out yet, so maybe I should. Spoilers through the end of this paragraph, okay?) Although I haven't yet cried when Fili and Kili die, I do feel that emotional gut-punch, and Thorin's reaction to seeing both his nephews fall is agonizing. And then, of course, when Thorin dies and Bilbo cries... wow. Words don't do it justice.