When I saw the Living Monolith, I knew I had to use it. It gave my players quite a beating. The Baga Yaga is another one I like that ended up being an interesting fight (the party cleric having to cast hold person on the confused fighters so they wouldn't turn and kill the party).
I am trying to throw them in when the encounter tables get stale, but I have no favorites - yet! The Shadow Dire Bear was fun.
My favorite piece of art is the Cinder Knight!
I like the Chucky Doll--the skull child
Creepy art, one of the best pieces I think.