The first thing that comes to mind is spiders. I get the heebee geebees just thinking about them!
Anxiety and panic attacks are another fear of mine. I would not wish them upon my worst enemy. They are one of the few things that ever made me feel as if I were dying.
Last but not least, the third thing that represents fear for me is small planes. I am just fine in large ones, but I cannot handle the small ones. While in Costa Rica I was flying in a Cessna that almost crashed. I have never and plan on never flying in one again.