These could be games that left a lasting impression on you, games that had stellar gameplay mechanics, characters that captivated you, games that you played tons of hours on, etc.
RTS by age: Dune 2, C&C, Tiberium Sun, Red Alert, Red Alert 2, WarCraft 2, StarCraft, Warcraft 3
Sim by age: Conway's Game of Life, SimCity, SimCity 2000, The Sims, The Sims 2
Strategy by age: Civilization, Civilization 2, Masters of Orion
RPG by age: Final Fantasy 2 (4), Chrono Trigger, Pools of Radiance, Eye of the Beholder, Ultima Online, EverQuest, Icewind Dale, Baldur's Gate, Baldur's Gate 2, Planescape: Torment, Fallout, Fallout 2, Neverwinter Nights, Morrowind
Adventure by age: Pitfall, Indiana Jones, Maniac Mansion, Monkey Island, Full Throttle, Sam & Max, King's Quest, Space Quest, Tomb Raider, Grim Fandango
Honorable mentions: Microsurgeon, E.T., AD&D Minotaur's Labyrinth, Golden Axe, Sonic the Hedgehog 2, Ecco the Dolphin, Eternal Champions, Android Pinball, Solar Winds, Detroit (not Become Human; it was a car making sim on DOS), Crusader: No Remorse