Half Life 2,
2004, a year where it shared headlines and attention with juggernauts like Halo 2, Grand Theft Auto: San Andreas, Metal Gear Solid 3: Snake Eater, World of Warcraft, Monster Hunter, Far Cry, Fable, Tony Hawk’s Underground 2, Killzone, Red Dead Revolver, DOOM 3, Ninja Gaiden, Pokemon Emerald, Prince of Persia: Warrior Within, Star Wars Battlefront, Spider-Man 2, Need for Speed: Underground 2, Gran Turismo 4, Unreal Tournament 2004, Metroid Prime 2: Echoes, Rome: Total War, Ratchet & Clank: Up Your Arsenal, The Sims 2, Counter-Strike: Source, and many other games.
2004 was arguably the greatest year in video game history in terms of video game releases.