The VOIDRUNNER'S CODEX is LIVE! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
At first, it was a tough call between the mindflayers and the zombies. Then I realized: Illithids eat brains because they need to survive. Zombies don't need no excuse to crack open your skull and suck on the moist bits; they just do it because that's what zombies do. Definitely zombies. Definitely.
Zombies have no festering contempt for you pathetic mortals. Gnolls don't eat your delicious brains (well, not specifically). Illithids it is!
Plus they're so much more powerful at the base level - they are simply better than you are. So some people think elves are the master race? There's a civilisation down below that eats elves for breakfast. Literally.
Zombies are my favorite, but vampires run neck and neck for second. I have to admit that most vampire books/movies/etc. are better than the ones with zombies.