The Witcher trilogy. I created an account for a giveaway, I believe it was Saint's Row 2, and then I saw the introductory sale. I had heard of the Witcher games but had little interest to play them; but I figured $30 for all three games was a good deal. I had also seen
RangerDave's review of the game and it convinced me to go for it. That one purchase was one of the greatest purchases of my life. I went from thinking W3 MUST have been overrated, as I didn't think there was any way it actually lived up to the hype... and then it did, though it did take a little time to fully sink in for me.
I was also very skeptical of CDPR based on all of the cool stuff I had heard about them. I had assumed that there was some sort of catch, yet they really proved themselves to me. In fact, that skepticism really helped with my perception around Cyberpunk: I went in with realistic expectations (instead of expecting impossible perfection) and as such, I LOVED the game. Of course, they're not perfect, but CD Projekt is definitely my favorite company in the industry right now due to my love for their games and the way they treat the customer (despite many of said customers being very bitter/hostile toward them over the silliest stuff).
What's more is that it led me to really start promoting/spreading the word about DRM-free and eventually, GOG became my main gaming platform.