I'm not sure about my "First coaster", most likely something at American Adventure, or Big Apple at WMSP, I can't remember too far back :?
My first "major" rollercoaster was Shockwave at Drayton Manor; which was surprisingly a big step for me back then; considering I had trouble braving myself to go on the smallest of coasters. And the fact it was Stand-Up was another factor which changed my view on rollercoasters forever, I was pretty much fearless after Shockwave.
Still to this day, riding Shockwave gives me anxious flashbacks from when I was 10 years old, shivvering in fear waiting for my first ever inverting coaster.
Shame I had to spend my first "major" ride on a rough, short, badly layed-out, bog-standard Intamin coaster which I have mixed thoughts about, yet I still have that ounce of nostalgia.