ULTRAKILL is the most fun I've had with an oldschool FPS in ages. The difficulty ramps up perfectly, it's super approachable for new players, and the skill ceiling for what you can do when you're good is absolutely insane. The soundtrack is also fire.
Clips like this make me want to do another playthrough: https://www.youtube.com/watch?v=UdHVt7cTwXc