Assuming someone had the mojo, and was foolish enough to summon the aspect of an arch fiend to the prime material plane; which one would you fear most?
Fierna from Phlegethos. She's actually the main villain of my current Castles & Crusades campaign.
I think it's not about the raw power that makes her scary, but her ability to lure and charm almost any creature. She's the one capable of collapsing the entire kingdom into anarchy without even showing her face. In combat she is very strong too - an unrivaled master of fire with power so strong that even creatures with fire immunity are not safe.
Her biggest flaw is her volatile personality. With how low-tempered she is, she's more prone to make mistakes when her plans don't come to fruition. So there's a light in the tunnel :)
Another good contender would be Mephistopheles, but that one is pretty obvious.