Alma in the first F.E.A.R.
The fear factor (ba dum tish) of this series got kind of watered down in subsequent iterations, but Monolith nailed using Alma to dial up the tension and atmosphere in their first outing. Drawing on inspiration from Samara from The Ring among other sources, there's just something undeniably unnerving about creepy children. Usually Alma would show up in longer drawn-out hallucinations, but they weren't afraid to indulge in a jump scare or two - that had me impulsively firing off half my shotgun ammunition. FEAR's fumble was that it tried to explain too much. Similar to how the theatre of the mind is often the most frightening, once all the curtains are pulled back and everything is tied up with a neat bow it lost all the mystery. But there was a golden few hours where Alma was genuinely scary
Second place goes to those ninja f***ers that jump down from the ceiling at you.