These are games that you can play over and over, EVEN when they become outdated. Sadly, not many recent games qualify for this list, although not because of nostalgia purposes, more so because developers focus on multiplayer more than they do singleplayer these days. But when you do find that one beautiful singleplayer experience that has as much love put into it as all the Battlefield maps combined, it's a joyous gaming occasion!
#5 Half-Life 1
Half-Life 2 was no slouch, but nothing compares to the claustrophobic adventure in Half-Life 1 and I can't wait for the remake with today's graphics that Valve will soon release. Maybe you had to be there to realize how awesome this spectacle was back in the day, but watching a story of a top secret laboratory test gone wrong and your forced escape to the surface made for one great story which was ironic considering most top stories come from the RPG genre. The mysterious man that follows you everywhere is still to this day one of the most original story elements in a shooter.
#4 Medal of Honor Allied Assault
The first PC game to cash in on Saving Private Ryan and the WII eating frenzy that followed. People probably remember the Call of Duty or Brothers in Arms series better but NOTHING compared to the Omaha Beach storming in MOH: AA. A truly novel level for its time and it set the tone for an entire experience that felt like you were in Europe fighting the Nazis.
#3 Deus Ex
Nevermind the sequel that you might have played on Xbox, this was the original smash hit and the best RPG/Shooter hybrid for its time. A buggy launch probably hurt it some but by the time it was finetuned, it held a special place in most PC gamers hearts. A cross between The Matrix, Illuminati conspiracies, Robocop and several other sci-fi themes you know and love, this game had it all, and a plot that allowed you to tackle missions anyway you saw fit, including different endings.
Most people prefer its spiritual predecessor System Shock but I feel today's art and graphics make Bioshock the superior version, and probably the only game of the last 3 years that can compete with these other oldies for phenomenal single-player experience. They're making this game into a movie and it's the first video game to movie production I would ever tell the movie studios to change nothing. Travelling through Rapture isa Jules Verne sci-fi adventure that few games can touch.
#1 Clive Barker's Undying
The essential horror game for the PC. Nevermind space colonies and mutants, the best horror shooter is an old fashioned mysterious mansion. With one of the most innovative magic/weapons fighting systems, this game was an adventure and a half as you tried to discover the terrible fate of a family doomed to eternal hell. Probably the final level was the worse, but playing this game with the lights off and the sound full blast was an epic experience.
System Shock 2, Half-Life 2, Far Cry, F.E.A.R., The Operative: No One Lives Forever, Serious Sam, Return to Castle Wolfenstein, Call of Duty, Jedi Knight II: Jedi Outcast, Jedi Knight 1, Duke Nukem 3D