I voted for
Balrog Vs Gandalf. Why? Who cares, but I'm gonna explain anyway
Eowyn Vs. Morgul Lord. Nice dio but I already have both statues and they fit perfectly together to replicate that scene, so why a (smaller) diorama? Only because of some more surroundings?
Treebeard with Merry and Pippin. Nice idea, but with the scale it would either be only part of Treebeard, or some VERY tiny hobbits. That wouldn't work either way.
Elrond and the Council. Good second for me, but with so many figures, either the figures would be tiny or the piece would be HUGE.
Frodo/Sam on Mount Doom. After a very disappointing diorama with Frodo and Gollum (I mean come on, what kind of environment is that? A piece of rock?
), this one would have to include a significant amount of surroundings to work for me.
Tom Bombadil singing. Tom who? Yes, yes... I know... he played a significant part in the book. But come on... We all know what GG did... easy pass for me.
Sam dancing with Rosie Cotton. Cute idea, but no thanks.
Therefor, Gandalf Vs Balrog was a very nice winner. If done right, it would be an amazing piece. Although, as some have mentioned: the size difference between Gandalf and the Balrog might be a possible problem.