1. Half-Life 2 (2004)
An astonishing game on every level, from the combat to the puzzles, the writing to the sound to the original and convincing design of its gameworld. But perhaps best of all was the fact that you could pick up a circular saw blade with the Gravity Gun, shoot it at a zombie's neck and watch its head pop off like an overripe, brain-filled coconut. As FPSs go, this could remain the greatest for another 10 years.