Half-Life 2 (PC, 2004)

Dr. Gordon Freeman is a physicist at Black Mesa Laboratories, or rather, he was before the experiment that went awry and created a resonance cascade, a portal that allowed monsters from another planet to infiltrate the lab.  Turns out that the intelligent beings of that planet were in cahoots with one of the other Black Mesa Scientists, Dr. Breen, and the experiment was part of a plot for him and the aliens, known as the Combine, to rule the Earth.  In the first Half-Life (1998) game, Dr. Freeman had to escape the alien-infested lab.  In Half-Life 2 (2004), he joins a resistance against the Combine and inspires a revolution as the one Free Man.
The odds are against Freeman, however.  Hardly any of the humans are willing to fight back against the fascist Combines.  These masked, humanoid aliens beat humans who don't follow orders and use frequent torture on random civilians.  Dr. Breen projects his likeness everywhere, with giant video monitors looming over the city streets, and TVs playing nothing but recorded messages (all lies) from him.  There's no point in trying to escape from the cities since what lies outside may be worse.  You start out weaponless, getting pushed around by Combine soldiers.  When they figure out who you are, they overwhelm you, but then you are saved by Alyx Vance, a resistance soldier and daughter of Dr. Vance (from Black Mesa).
Meeting up with the remaining Black Mesa scientists (now calling themselves Black Mesa East and working in hidden makeshift facilities), Dr. Kleiner and Dr. Vance, you receive their prototype gravity gun.  With this, you can pick up, carry, and launch almost any object, making anything you can find ammunition.  There are other weapons in the game (such as the series-standard crowbar), but the gravity gun is your most important tool.  Half-Life 2 uses the Source engine, which allows for realistic physics.  Many parts of the game require puzzle solving with physics.  One such puzzle has you using barrels to hold up a plank of wood underwater so you can walk across it.
The storytelling in this game is perfect.  most games (before and since) relied on cut-scene intermissions to relay expository and story information.  In both Half-Life games, the story is told in scripted sequences, where the player retains control of Freeman while characters talk or events play out.  It is one of the many ways these games are amazingly immersive.  Another way is that the world you travel in feels completely real.  The physics behave fairly realistically, and almost everything can be interacted with.  In Dr. Kleiner's lab, there is a miniature set of transportation modules, and you can play with it if you want.  When driving down a highway (there are two driving sequences) there are houses you can stop in.  Some of them tell stories without any dialogue.  You might find a pile of boxes, and under it two dead bodies:  one of a human and another of an alien monster.
This game maintains a highly unsettling atmosphere all the way through.  After Black Mesa East is attacked by the Combine, Freeman escapes by going down the blocked-off underground route to Ravenholm (Alyx tells you beforehand: "We don't go to Ravenholm anymore...").  Ravenholm is a city overrun with headcrabs, alien parasites that latch onto the heads of their victims and turn them into zombies.  This part of the game offers possibilities for all kinds of scares, shocks, psychological horror.  In another part you must go through Nova Prospect, an insane asylum-turned-Combine research hospital, where experiments are performed on humans, turning them into grotesque beings.  The corridors are dark.  Sounds of machines and voices echo from unknown origins.  You feel the presence of another being, known only as the G-man, whose origins and motivations are still unknown.  He has appeared to Gordon before in dreams and hallucinations, but has never manifested until Nova Prospect, where you see him on surveillance monitors.
Half-Life 2 is one of those rare games that can immerse, tell a great story, and still be fun.  Although it is at its core, a first-person shooter, there is no other game of its kind like it.  Its combination of incredible gameplay, chilling atmosphere and original storytelling insure that it will have a longevity much greater than that of its contemporaries.  Even almost ten years later, it is better than most shooters that come out today.  Half-Life was only the early sign that Valve (see also my article on Portal) would have great things in their future.