Spiders! Seeing a big one makes me ill - skin gets clammy and I get nauseous. I have to leave until someone 'disposes' of it. If there's nobody around, I'll spray it with Spider-Ban ... which is usually accompanied by me screaming at the same time!
To a lesser degree - heights (can't seem to get past the 4th rung of any ladder) and being unable to touch the ground when swimming.