For me it was definitely DOOM 2! Miles ahead of anything else I had played before.
Morrowind, by far. I still remember the sense of freedom and exploration I got
Also Morrowind. The systems of that game blew my young mind, and I was far too dumb to notice most of the jank.
I was too young for Morrowind and started at Oblivion, but yeah, it’s the Elder Scrolls games 100%. At the time, to me (age ~9), gaming was jumping and gunning around blocky worlds full of fake doors and imagining how cool it would be if GTA felt like an actual world instead of a blocked-out setpiece full of people whose only thoughts were to walk around, drive, or fight each other.
I started Oblivion and it was insane. I could go in nearly every house, I could have conversations with everyone, I could walk around picking up whatever objects and stealing stuff, then break out of jail when I got caught, I could get inducted into an assassination cult (even if I was really bad at lockpicking and struggled to get in the front door), etc. It was mindblowing and those sorts of features are why I prefer Bethesda titles even to these major titles everyone loves like Witcher 3.
Same. One of my friend’s dad played all the old school DnD games and what not. I remember going over one day and seeing him play that, and when I asked him he was showing me a bunch of things with the open world and the characters. As soon as I was able to get it, I did, and I put in so much time into that game.