Kerbal Space Program. Any Civ Game. I suspect that I'll be playing Fallout 4 for a while, even though it's really not that good: There's something distractingly immersive about wandering around the Commonwealth listening to the radio. Similarly, Skyrim.
RTS/TBS - Civilization (any) - Age of Empires II (now remastered on steam) - Rise of Nations
Roguelikes - FTL - Rogue Legacy
World Builders - Sim City 3k - Terreria/Minecraft
Fighters/Shooters/Hack-and-Slash - Day of Defeat - CS 1.6/CS Source
Puzzles - Portal/Portal 2 - Puzzle Quest
Great Cashdumps (They have TONS of DLC content) - All games by Paradox (Victoria, Hearts of Iron, EU, Stellaris)