I reckon Nazgul: the Dementors can suck your souls out, but they have to catch you first (of course, this doesn't cound for much if you have Harry's little problem, but I digress); whereas it said somewhere in LotR, of the Witch-King, that "even the rumour of his coming" made people really afraid, lose all hope, leg it to the nearest bed so that they could hide under it, etc.
Fear is a powerful weapon. Especially as people run slightly slower when they've wet themselves (fact).