For me it was XCOM Enemy Unknown when it was given away as a Freebie for participating in the 2013 Golden Joystick Awards. At first I was unsure if the game was something I’d like because the screenshots I’ve seen of the game at the time made me think the game was a First Person Shooter. But when I booted it up for the first time and saw first hand that it was a turn based tactics game, I was hooked instantly. I did had to learn about the game’s cover mechanics, but as soon as I learned the ropes so to speak, other turn based games that don’t have a cover system like the Super Robot Wars games no longer felt that difficult.
I thoroughly enjoyed XCOM EU that I bought all three of its DLCs right off the bat without even waiting for a sale discount. I also bought XCOM 2 at launch. And Chimera Squad at launch too because it was cheap and was also given a 50% discount before and immediately after launch.