Fear.

The fastest emotion to reach the brain

to create a fight or flight reaction

Fight. 

Fear causes a person to attack

to rid of whatever caused the fear.

Flight.

Running as fast as you can

away from fear and danger. 

Dark.

What I fear the most.

You never know what's in it;

You never know who is there

the dark hides everything from you

And it will consume you

Just as fear itself.