Day 18-Favorite Monster (Immortal/Outsider)
Devils, although I don't use them enough. I find the Devil/Demon dichotomy a little strange. I think the medieval idea of hell covers both types, but D&D loves its metaphysical organization so we have Lawful and Chaotic ultimate evil. I guess I just always wanted to use a pit fiend but don't get high level very often. I should fix that.
Day 19-Favorite Monster (Elemental/Plant)
Your basic elementals are pretty cool. They're the only thing in the AD&D Monster Manual that goes into detail about summoning, so they're the template for summoned monsters. And way cooler than a celestial dire badger.
Day 20-Favorite Monster (Humanoid/Natural/Fey)
Broad category. High elves, or 4E Eladrin. Because I can use them as a template for Sidhee, Tuatha de Danaan, Alfs, whatever mythological fae race I'm into. And the pupil-less eyes is just like elves in The Broken Sword.
Honorable mention to Displacer Beasts, because they're badass.
Day 21-Favorite Dragon Color/Type
Green Dragons. Classic dragon color, and they're sneaky. Mythological dragons are usually serpentine and poisonous, so they're similar to that. And I seem to have a bunch of random green dragon minis, so I'd like to put a family in a campaign and have every mini be a different member.
Day 22-Favorite Monster Overall
Its not D&D, but something I've stated up a few times: the Nuckelavee. Scottish water fae/demon. Its like a centaur that's been burned by nightmare acid. Skinless, with blood and black ooze dripping everywhere. Flippers on its feet, long gangly arms with huge claws. An oversized head that rolls around because its neck can't support it, and a huge eye that burns and poisons everything. (Some versions say its basic form is a horse with a half human body coming out the middle of its back, but I prefer the centaur-like version.)