Reality: Stories that are often fiction (not real) often tend with reality, but play with it to make it feel as if the protagonist is dealing with usual life. However, some stories deal with actual reality. What do I mean? I mean every single negative and positive thing in the world, whatever is left of the positive ones, anyway. You can deal with corrupt politics, abusive parents, suicidal thoughts, everything that really happens on this planet. The true reason why it is so utterly terrifying?
BECAUSE IT IS NEAR IMPOSSIBLE TO CONTROL.
Unlike physical conflicts, reality is a complex problem that affects everything it can touch, which is quite literally everything. It is a monster with its arms of plague infinitely spreading from one side of the world to the other. You cannot escape. While it may be possible to sever one of its arms, another will just grow back, so you're better off conforming to it.
Blankness: Nothing. A void is where you are, ominously waiting for whatever to happen. You don't want to know what's after you, but you don't want it to get to you. you think you hear a noise, a shuffle of someone running past. Who is it? A murderer? A monster? A deadly prank by one of your friends? Or perhaps...something else more terrifying. Suddenly, your eyesight begins to wane and fade, and eventually you are on your knees. You look down, only to see to your horror a blade that has perforated your chest. You fall face first onto the floor, and a pair of hands lift up your head. Before you black out, the black figure smiles as would a demon and whispers the final words you will hear:
S W E E T D R E A M S.
So, yeah, I chose the second thing I fear the most as "nothing", since you don't know anything. Will you die right now, or later? Are the noises in the woods a bird or a beast out for your skin? As quoted by Alfred Hitchcock below:
"There is no terror in the bang, only in the anticipation of it."