Half-Life 2, an ageless classic of PC games, the title that put Valve on the map, the reason Steam exists today and one of the most ground-breaking titles of it's time. I'm going to say right now to those who haven't played Half-Life 2 to do so immediately, as it is an incredible game that to me has triumphed over the tests of time and remains high on the list of video games that can never be bested.
Half-Life 2 is just about perfect in every regard: from it's carefully forged story to the perfected pacing, from unforgettable characters to a steady challenge curve. The entirety of Half-Life 2 is a journey leading the player full circle in their quest to liberate humanity from their omnipresent multidimensional conquerers, the Combine.
But if the game wasn't already grand enough, Valve gives the player ultimate control over their physics engine with the highly beloved Gravity Gun (you can call it the Zero-Point Energy Manipulator, if you really want to), a weapon that has never been equaled in any game since. The Gravity Gun allows you to pick up objects from the world, like barrels, sawblades, crates and anything you can move and use them as protection against enemy fire or throw them at your foes in a satisfying method. This is even encouraged by the developers, for the level immediately after you recieve the Gravity Gun you are placed in a zombie-infested town with limited ammo and plenty of sharp, explosive or otherwise deadly-when-hurled-at-100kph objects. In other words: A playground for your new best friend.
So if you're a recent gamer looking for a timeless classic or a veteran tired of all the two-weapon inventory crap in modern games, I sincerely and whole-heartedly recommend Half-Life 2 for an experience that has never been topped.