For me it's gotta be the battle of Pelennor Fields. I mean, it had everything: A siege, flaming catapults, a giant wolf-shaped battering ram, Gandalf telling the soldiers of Minas Tirith that they must hold their ground no matter what comes through the door a second before three very angry Mordor Trolls wearing very spikey armour bust through, a massed cavalry charge, (with awesome, fitting music in the background, even without Amon Amarth) a dragon, giant elephants with four tusks, really awesome moves from the heroes to bring down said elephants, (Legolas on the haldar, the shot into the Beastmaster which takes down two Mumakil, Eowyn slitting the ankles of one) ghosts, and pretty much everything else.
Helm's Deep is a close second though, tying with the Last March of the Ents.
Moria, Shelob and Amon Hen didn't actually have the whole "two massive armies" thing going for them, so they lost a bit there. If a company of Wood Elves and/or Galadhrim had been escorting the Fellowship, then it would be pretty cool because of the whole woodland fighting part of it.
This is a warhammer forum. Everything is a military operation.
I have spent the last 25 minutes doing reconnaissance on my kitchen for operation: Cup of tea.