Sony PlayStation. That's the original PlayStation, or the PS1.
More specifically, the video game Final Fantasy VII on the PlayStation.
Before that, I had played games on NES and Genesis and enjoyed them. I played plenty of classic games, like Super Mario, Sonic, Earthworm Jim, and even Comix Zone. They were great games. But they were all pretty simple games with simple stories, gameplay, and no character growth. Most of these you went right, jumped, and defeated enemies. Outside of gained new weapons, or using power-ups, you ended with the same character you started with and saved the day.
Then I played FFVII. It changed my perception of what games could do. You had actual characters with personalities. The story wasn't simply that bad guy does bad things, probably kidnaps someone, and you have to stop them and save the victim. And, while I know there were other games that did it, the gameplay was different from any game I had ever played. It changed my perspective of what games could be.
And I soon was ravenous for other RPGs. So I played Lunar, Legend of Legaia, Star Ocean, and many more on the PlayStation. That's why the PlayStation made me a serious gamer.