What video game introduced you to your favorite (or one of your favorite) genres of games?
For me, it was The Banner Saga that got me into visual novels and strategy RPGs; without it, I'd never have enjoyed Steins;Gate or Fire Emblem Fates.
For MMORPGs, it was Air Rivals/Ace Online (yes, the one that shut down last year) that got me into MMOs. The idea of playing a game that simply didn't end after fifty hours of play, but continued with content updates and a wealth of activities to participate me, as well as seemless multiplayer whilist also being able to play by yourself when you want, was what led me to Star Trek Online and eventually WoW. I eventually left ACE when I realized how shallow and low quality it was when compared to Cryptic's and Blizzard's offerings, though ACE still remains nostalgic for me.