Six-ish years ago I would say Overwatch. It was my first online multiplayer FPS and it fosters a lot of skills. Teamwork, communication, mechanical ability, game sense, ability management, managing tilt, etc.
Too bad Blizzard decided to stop new content for Overwatch 1 for years, only to reintroduce Overwatch 1.5 with an upgraded battlepass and cash shop monetization scheme. I don't get how people are still playing after what they did to it.