There is no single "best" game for hardcore gamers because preferences vary widely. However, hardcore games typically share these traits:
Counter-Strike: Global Offensive, League of Legends, Dota 2, Rainbow Six Siege, Apex Legends
Dark Souls series, Elden Ring, Bloodborne, Disco Elysium, Baldur's Gate 3
StarCraft II, Civilization VI, Factorio, XCOM 2, Kerbal Space Program
The ideal game depends on the type of challenge you enjoy. What genres or mechanics interest you most?