This one is easy and will be very short. I'm way over my posts-per-month ratio by now. And 9 rather uninspired Favorite Monster Questions start to make this a challenge after all. Well, I'll make this short. Favorite Outsider:
Check out the Wikipedia page for Orcus.
Or, better yet, listen to some Corvus Corax:
Neo-medieval music with an orchestra and authentic instruments (more information from the Wiki). Enjoy!