Drop Tower.
I used to have a paralyzing fear of heights to where I wouldn't even get on an observation tower. My ex thought it would be cute to drag me on it. By the time we got to the top, I was in tears and screaming every cussword I knew upside down and backwards at him.
Millennium Force also scared the garbage out of me on my first CP trip, because I was still afraid of heights at the time. The highest coaster I'd been on before that was Vortex, and when it hit me how high up we were going, I had a panic attack on the train as it was leaving the station. I even closed my eyes at the top of the hill for a second or two.
MF destroyed my fear of riding anything after that, except Skyscraper, which darn near had me wishing I had a diaper on my first ride. I was fine until my friend who was a ride op on it took my seat and spun it in a series of 360s on my first ride.. I was screaming cusswords at him.
The hilarious thing is, all three are now topping my list of favorite rides.