Scorpions disturb me deeply, on several levels: not only are they freaky-looking (completely unlike anything else), they tend to set up the worst, most horrible ambushes. To wit: I was out camping in the desert when I was a kid. Woke up, grabbed my boots, only to have my dad say "WOAH WOAH! WAIT!!! Shake out your boots before you put them on. Scorpions are attracted to moisture and love to hide in people's boots."
sure enough, there was a scorpion in each of my boots!
hatehateHATE the freaky, alien-looking fuckers now.
EDIT: Black Widow spiders are a close second. They just look evil. When I was a teenager, I worked as a cable installer in new subdivisions (this was in the deep South of the USA). The guy who trained me taught me that the dark and damp insides of the street cable box terminus (where you connect a subscriber's home to) is the perfect environment for them, and when you opened those boxes, you better be wearing heavy gloves and be ready to jump back. Sure enough, at least half of the cable boxes I opened working that job had black widow spider about the size of a half-dollar in them. I got very adept at hosing out those boxes...