1) Octopuses. Those things scare the hell out of me. Don't ask why.
2) Jellyfish. Got stung once, painful and highly freaky. Won't go near the things since.
3) Squid. Especially the big ones that live at the bottom of the sea. They terrify me beyond all reason.
4) The dark. Well, not so much the dark, but whatever might be in it. A vivid imagination coupled with reading way too many books does that to you.
5) Small confined spaces. I'm a total claustrophobe. Enough said.