It's difficult to reasonably give a product a perfect rating- nothing's perfect, after all. That said, Half-Life 2 toes that line so perfectly it sets the 'perfect standard' against which all other games are compared.
Picking up after the events of the first Half-Life, the G-Man (a mysterious alien force who comes in the form of a businessman) drops you, Gordon Freeman, physicist and supersoldier extrordinaire, into a train filled with dejected and tired civilians dressed in Orwellian uniforms. Welcome to City 17- one of the few remaining human centres. You see, the Combine (a group of aliens who teleported to Earth thanks to your actions in the first game) have taken over the planet, and are now taking steps to stamp out humanity- chemically supressing reproduction, slave labour, recruitment of civilians to form brutal police squads, the list goes on. I won't ruin it, but things quickly go down-hill for Gordon and before you know it you're fighting for your life.
On a technical level the game was, and still is, light-years ahead of competing games. Maxed out, the graphics were mind-blowing... and they still rival anything any of the consoles has offered, almost 5 years later. However, it still plays very well on older systems and hass been continually updated to make use of the latest technological developments. The game integrated the Havok physics engine, which gave nearly everything in the game its' own realistic physical behaviour models and added a whole new facet to combat- baddies shooting at you? Hold up a dumpster lid and use it like a shield to advance. Grenade thrown at you? Grab it and throw it right back. The sound quality, replayablility, moddability, etc, are all up to the same standard as the rest of the game.
If you buy one PC game this year- or ever- make it Half-Life 2. The community is still strong after 5 years, and will continue to be for a little while yet. When you buy Half-Life 2 in store it also comes with Counter-Strike and Half-Life: Source or Day of Defeat: Source. Great games, great value, you can't go wrong.