Heights......without a doubt. It's actually a morbid fear. I become light-headed and nauseous, simply LOOKING up and imagining being that high up. I don't know when or how I developed this particular phobia, but I've not been able to overcome it.
I have vivid frightening memories of the waves of panic I have experienced, on occasions when I found myself looking way down from where I happen to be standing. The severe physical reaction includes rapid heart beat, hyperventilating, I've even burst into tears.
Any other minor phobia I may have, pales in comparison to heights.......Can you imagine being in Paris, France and not going up the Eiffel Tower? I couldn't even watch my family reach the top!