HoloCure out of all things. It's a more advanced Vampire Survivors clone centered around HoloLive vtubers. Being so simple yet cute and addictive is what made me hooked. Even now I still play once in a while on my breaks, althought there is nothing to surprise me left in this game.
Previous to that, I remember being absolutely blown out by Fallout 1. I'm young enough not to be on the wave of games of that era and the game design amazed me. I look forward to playing the 2nd game fo the series but I will have to do some tinkering since I'm Linux-only now.