Gemini at Cedar Point.
Prior to that, I was so scared to go on anything even remotely high (this was 1985). Oddly enough, I would go on Demon Drop, but not Gemini. Finally, my sister kept nagging me, telling me how awesome Gemini was. I was terrified, but I went on it anyway. After that ride, I was hooked. I still got scared of a few coasters, but I wasn't phobic, and I'd ride anything. I was very nervous for Magnum the year it opened, but I loved it. Same with Mean Streak.
The last coaster I remember being nervous (scared nervous, not excited nervous) was S:ROS (SFDL) in 1999. After that ride, I've only been excited to ride new rides, not scared. I didn't even think about MF being so high..I was too pumped up to even be remotely scared of it. However...if I ever get a chance to ride X, I forsee some fear there. that first drop looks like it may be the scariest thing in coasters. (The Gio tilt coaster would be pretty terrifying too...but I'd be first in line!)
Jman