Named Game of the Year by over 50 publications, Valve's début title blends action and adventure with award-winning technology to create a frighteningly realistic world where players must think to survive. Also includes an exciting multiplayer mode that allows you to play against friends and enemies around the world.
Definitely one of the greatest, genre defining games of all time. It may be getting older but that fact does not decrease the playability one bit. And when a game survives the time trial as well as Half-Life, giving it anything less but the max rating would be a sin.