StarCraft: Brood War. I played it so much in my teens – decent single player, amazing multiplayer, and even better custom maps that would just consume my days after school. For me, it's the best game made to-date.
I don’t play it anymore, but I do still watch pro matches/tournaments; even now, 25 years(!) later, the pro scene is really healthy and dominated by Koreans (much moreso than StarCraft II).