Little bit embarrassing I suppose, but for me it was MP Xpress at Movie Park. Before that, I quite literally hated going on any rollercoaster (even some kiddie ones), but since MP Xpress felt painful for reasons I didn't anticipate (thanks Mr. Vekoma), it convinced me that the reasons why I was afraid rollercoasters were bull**** all along, since I survived this mess of a Coaster just fine.
I knew a couple of things about Coasters back then, even that people hated SLCs, but for some reason I always thought it was just because there are so many of them and people were tired of riding the same thing, so I didn't see any issues back then in starting with that. Oh well.
Then a couple years later, I postponed riding Silver Star to the 2nd day of my trip to Europa Park, but after that, it really was only Highlander that got me close to shatting the bed. Glad I did it though.