Favorite Monster (Immortal/Outsider)
Rakshasa. Even back to the original AD&D Monster Manual (before they were called outsiders), the rakshasa freaked me out.
I loved the idea of a worldly and civilized fiendish creature (perfectly epitomized in the original MM illustration by the iconic Dave Trampier) that would focus its destructive, carnivorous urges through cunning and malice, as opposed to the chaotic destruction of demons and the otherworldly malevolence of devils. The best expression of that probably came in the Eberron books I worked on for D&D 3e, in which rakshasas played a huge part in the past history — and possible future destruction — of the game world.
Also (though it’s unfortunately not rendered as clearly as it could have been in the Trampier illustration), the rakshasa’s hands are on backwards. That’s pretty badass.