The Witch-King of Angmar - Of course, this has to be
Morgul Queen. Just watch her take out those Silm-Sues! Scary!
The Balrog - I'll say...
burrahobbit, because of how he can get his point across with so few words. Plus, I hear people are scared of him.
Saruman - Naturally,
Sharku claims this role. He seems to know far more than is good for him. (You know, I've never been able to solve the crosswords he's made...)
Shelob - Why do I see
dancing spawn of ungoliant as filling this spot? Maybe because the thought of Shelob doing a two-step is just so darn amusing! ^ ^
The Watcher in the Water -
Mithalwen, as she has a tendency to attempt to eat people who annoy her.
Assorted Orcs - Everyone who's left!

Just kidding. I'll give this spot to
Morsul the Dark, in holding with Eomer's comment.
Abedithon le,
~ Saphy ~