There is not much to specify for the question, its really simple: Which title is the first game you have purchased on Steam? Was it a good decision? How much did you pay? When was it and what device? You don’t have to answer all questions, this is just a fun discussion to have.
You can check your history: https://store.steampowered.com/account/history/ Edit: Some in the comments pointed out that the purchase history doesn’t go back all to the beginnings of Steam. So if you purchased something earlier, it won’t be listed on your account.
(I reply with my game too.)
Thinking about it a bit more, I now recall I had multiple steam clients installed in Wine to start them up with different Wine settings, because the games didn’t run with the same settings.
But to see my first game, I may have to ask my wife, because she had Visa before me.