1. Orc
2. Ogre
3. Cloud Giant
For this section, I'm lining up my monstrous humanoids: Human-size, large, and huge.
4. Dragon
They're iconic. I already default to virtually all of my dragons (regardless of color) being fire-breathers.
5. Skeleton
6. Incorporeal Undead
Here we line up the undead -- one corporeal, the other incorporeal. I'm not sure which incorporeal one I would select. I might end up creating a blended variety that would stand as a stronger, iconic tool.
7. Giant Spider
I only had room for one monstrous vermin on my list. Spiders are cool and creepy, so I decided to let them make the cut.
8. Werewolf
I wanted something that could be "normal human, but monstrous". It was either vampires, werewolves, or doppelgangers. If I was looking for a stronger theme, I might go with vampires (to fill the world with undead). But, in the interest of a slightly wider palette, I decided on werewolves being more versatile than doppelgangers.
9. Dark Elf
A civilized counterpoint to the uncivilized orcs. And they'll be filling up the Underworld and be responsible for creating the abandoned underground cities since this setting has no dwarves.
10. Demon
Not sure which exact demon I would go with. Torn between Mephistopheles and Cthulhu as the archetype, but there's a definite need for something "vast and evil and powerful from beyond the boundaries of the world as we know it".