Qlippoth - Great concept and execution, wish I had the occasion to use these guys more.
Oni - Enjoyed the idea of evil spirits as extreme hedonists.
Cyclops - Really dug the history of Ghol Gan's collapse.
Ghorazagh - Awesome and eerie replacement for the iconic floating orb monster.
Humans - Usually end up being the most monstrous foe my parties face in my games.