Marathon Infinity - The whole Marathon trilogy did a lot for defining the story-driven FPS (as did System Shock), but since the first and last title were Mac only for years, they didn’t get the credit they deserved among the pantheon of FPS greats.
Superbrothers: Sword & Sworcery EP - For years, whenever someone asked me what the must-have game was for iOS, this was always my answer. It shows up on a fair number of iOS lists, but doesn’t get the same level of recognition on PC. One of the most well-crafted experiences ever.
Clash at Demonhead - Despite having an Easter egg in Scott Pilgrim, this NES game is largely forgotten. It was one of my favorites in my youth and I am always surprised by how few people have played it, let alone finished it.