What monster do you always remember from playing in an RPG?
Was it terrifying?  Powerful?  Huge?  Otherworldly?

My favorite that always quickly comes to mind is the Mind Flayer from older versions of D&D.
They are smart, conniving, and love to eat your brain.

Second choice right behind that would be a beholder from old versions of D&D.
All those spells it can cast from its eye stalks and main eye are devastating.