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.)
It should work for pre-2008 because my first purchase shown is a digital copy of the Orange Box in 2007. Did you buy it boxed? It doesn’t show my boxed copies of HL2 or the HL platinum collection I’d bought retail before that point.
It’s possible that all my games before that were physical copies. But I could have sworn I bought stuff before on steam directly