I'm not a big fan of spiders, but I can usually deal with them.

Centipedes and millipedes break me. They flick some sort of switch in the base of my brain and I lose all rationality. I never used to think anything of them and then I saw one in real life, and something about it sent me completely round the bend. It ran across my desk so I dressed myself in gloves and boots and trout like I was going outdoors in a blizzard in Winterhold and then I spent 45 minutes hunting for the thing. Eventually Murphy found it, which makes him the most heroic cat in my book.