Day 18: Favorite Monster (Immortal/Outsider)
This one should be another obvious one. Demons. I have always been fascinated with them both as a subject and as a monster in *D&D. Unrepentant engines of destruction or vile schemers and tempters. What's not to love? When I am playing a Paladin then I love nothing more than to fight against them. When playing a witch I love summoning them and binding them to gain infernal knowledge and boons.
Plus, as they say, they are legion I will never run out of ideas or uses for them.
There is the argument that sometimes they are played like nothing more than high-level goblins with magic, I think there is room for those kinds and other kinds of demons as well.