I'm probably most afraid of myself. I often get psycotic urges towards people for little or no reason, and I'm afraid that if I ever loose control of myself I'll end up seriously hurting someone.
I'm also afraid of the dark, a bit. I don't like to sleep without a little bit of light, or at least a door or window open.
I also suffer from minor claustrophobia. I hate being trapped in a place thats to small or cramped to allow proper movement.
Linked to the above is my irrational hatred/fear of really 'stuffy' places. I can't bear rooms where it's too crowded an hot and there doesn't feel like there's enough air.
And for the fifth, I'll have to go for a classic: insects. I'm fine when their outside, but I hate having insects inside the house, especially in my bedroom. I the idea of them crawling on me in my sleep will sometimes keep me up for hours.