I believe someone mentioned this, but I would say humanity's worst fear would be ourselves. Just think, would you, where you live, currently go outside between the times of 10:00p.m. /to/ 3:00a.m.? I wouldn't.
But ya, I think our worst fear currently would be ourselves.
I think its probably war in general or the threat of war, after all, it was largly the treat of terrorism that got an idiot like George Bush back into power.
I think Humanities greatest fear would be EXTINCTION because we're always struggling to stay alive, we never let anything take it's own course now unless it makes sure we live.