For me: Tribes RPG.
https://tribesrpg.org/
I played Tribes, the team-based FPS, and fuggin loved it as a kid. Disc Launcher OP. Shazbot!, etc.
But then I saw this mod for Tribes RPG. You WHAT mate? First person RPG? Playing with others on the same server? Sign my 12 year old self up. The graphics were awful. The gameplay was terrible. But there were quests, monsters, and a character progression system.
When Iksars finally made it to EQ (my friend showed me the box art for Ruins of Kunark) I migrated to EQ - and haven't looked back since.
But if it wasn't for Tribes RPG wetting my whistle, and showing me what was possible with RPG games - I would probably not have had the itch to play EQ.
What game was it for you?