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.)
First game was Portal, but I got it free when Portal 2 came out. First games I spent money on and got on Steam were a couple of Humble Bundles (Introversion, Egosoft, EA). My first purchase actually from Steam was Goat Simulator.
First purchase on Steam was the Goat. xD Played it too, I think I got it through Humble Bundle. Man good old times.