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.)
I got my Steam account through a HL2 disc too!
I remember being very excited when Steam added a new game that was not made by Valve. It was, at the time, an interesting experiment in digital distribution outside a big publisher. “Rag Doll Kung Fu”, a shitty click-driven fighting game.
Early on, I never imagined they’d get big names like Call of Duty publishing there, but it happened sooner than I expected.