For me it was an old Mac game called Realmz. It was a sprawling party-based RPG sandbox that introduced child me to modern fantasy, turn-based tactics, and many other concepts that appear in genre-similar games across various mediums. I thought that its later incarnations were a step backwards, chasing shiny features that came at the cost of good mechanics and art assets (relatively speaking), which sadly made revisiting the game with an emulator less fun than it could have been. I'd love to try my hand at making a spiritual successor to it, but I couldn't make art assets to save my life...