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/
(I reply with my game too.)
2010 Torchlight, then Puzzle Quest three times, and then Terraria. Torchlight was for scratching the Diablo II itch, but it didn’t really do so for long. $20. Puzzle Quest was pretty addicting, so I gave two others their first hit. $1.50 for each one. Terraria was $10 and I’ve been playing it on and off for 15 years now.
Torchlight 2 was way better. I actually have far more time playing that than any Diablo game.
Torchlight was my first steam game too! Got it at the end of 2009 during a Steam sale, 75% off.