I GM a lot, and one thing I dislike is old monsters. Monsters are great out of the book, but when you get to the Big Bad, you need a little creativity. What do I mean by this? Well, for example in my Warhammer adventure I introduced my own creation: Chaos Zombies. They were pretty cool, their attack would come from a pustulant organic limb with teeth attached which burst through the zombies neck; it shoots about 2 meters across the room to inflict their diseased wounds. Why did I feel the need to add this element? Let me explain: my players have, at most, been roleplaying for 20 years to, at least, 8 years - you name it, they've seen it. However, I don't just add new monsters to give them a bit of a change, it's to enhance my game because they don't know what it can do or how to defeat it, and that's exciting. Also, it's hard for them to keep out of character information out of the game. Does anyone else do this?
Also, my most recent monster is one of the silliest: The Flying Spaghetti Monster (The image I pinched for it was from http://www.venganza.org/
about the Kansas School Board issue). It has some cool psychic attacks!