For me it's heights. But I'm not phobic, I just suffer from severe vertigo.
I have no problem climbing over balcony railings, dangling from ropes high above the ground, but if I stand at the edge of a 10 foot drop without my hand touching something I lose all sense of balance. Same thing happens with stepladders.
I have no idea why. I used to jump off the roof of a garage onto a mattress when I was a kid, do flips off into the snow, and jump off the lip of a quarry into the water below. But I can't just stand on the edge.