Well as a traditional fear, I've always feared dying unknown. Knowing that you will die and your life on here has been completely meaningless to anyone else is quite a scary thing. That's quite a scary thought for most humans though as many would love to think they have slight importance in this world or at least to another person in general.
Now as an ideological fear, I fear those or that which has enough power to assert lies, opinions, etc as the whole truth. Essentially those who claim to know the truth really freak me out. It's quite a scary thought knowing that someone could completely make something up, have enough power and reputation to assert it as the truth, and manipulate people thereafter based on this myth or lie. That is something I fear greatly and seek on a daily basis to fight against.