There's all these horrible spells, but honestly? It's the simple ones which could be the worst. The one in my mind right now is Fear.
A spell which causes you to feel utter terror, such that you are rendered helpless as you run from your worst nightmares. What does that spell make you see? Feel? Does it conjure up nightmares specific to the target, or does it just make them feel complete fear?
The idea that someone could simply waggle their fingers and bam, I feel terrified... Well yeah. It terrifies me. :P