Without a doubt - Mind Flayer.
I have not yet seen a player that wasn't creeped out if the HINT of a mindflayer was in the advanture somewhere - much less a whole COMMUNITY of the things. It's simple to play them as alien - they are unfathomable, they control your thoughts, they can eat your brain, they can send slaves of weak mind but tremdous strength against you, they can eat your brain, they have communities to back them up, they can eat your brain, and they after they eat the afore-mentioned brain, they steal your darned BODY!
Even before Bruce Cordell created the Illithiad, the Mnd Flayers were scary as heck all the way from the monster manual. The 2E Monstrous Manual made them even viler.
(BTW, if Bruce Cordell ever ends up reading this thread - I want you to know that the Illithiad is the one and only D&D work that has scared the hell out of me, and imprinted the nightmarishmess of mind flayer in my mind's eye forever.)