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 perfectly understand not caring for the hassle to get games top work back then.
I remember when the humble indie bundles were new, it was absolutely amazing to get a couple of games that were actually playable and fun. World of Goo was by far the best game I had played on Linux when that arrived.
That reminds me, my favorite Linux game I played before Steam on Linux was an Open Source game Teeworlds. I don’t remember if I played anything proprietary on Linux at that time (besides emulation off course).