Chiming in with Starcraft 2 (and 1). You can run it through Lutris like I set up for my friend or Steam which I use myself. The game runs great, it's highly polished, and still has a big community and pro scene. Somebody here complained about getting stomped in competitive, but both games use an MMR system so after some placement matches you'll get paired with roughly even opponents who are just as bad as you are :) There's also a (tiny but slowly growing) Starcraft community here on Lemmy and many on Discord. There are also more casual team games and co-op commander games as well as many popular custom game types/maps. One of my favorites is Battle Poker where your cards are units and the winner is determined by an automatic battle between player's units who don't fold.