Half-Life 2 forever changed our expectations for what a first-person shooter could be. Its richly imagined world and wonderfully paced gameplay is a delight, never letting up and brimming with invention. The Gravity Gun is obviously the poster child of Half-Life 2, turning each environment into a tactile playground in which you can create improvised weapons and solve basic but clever physics puzzles - and its importance can’t be overstated - but there’s an awful lot more here. We’re introduced to Alyx Vance, a supporting character with a rare warmth and intelligence. We’re transported to an iconic city, where Combine barricades loom with grim authority, and Striders stalk the streets with an otherworldly menace. We get to set Antlions on our enemies and in which we play fetch with a robot Dog. In short, it is a truly memorable piece of game design.