10. Half-Life 2
How many people do you come across where upon any mention of Half-Life 2 get they get that steely look in their eyes as a devout "I must impart this game to you"-type stare-down unfolds, with them uttering something along the lines of your life not being complete until you've experienced it? It's all for good reason though, Valve's second foray into the world of Gordon Freeman and co. remains their finest work to date - and that's some high praise for a developer when you've got Portal and Team Fortress 2 under your belt - but it remains that hardly anyone will seek out the first Half-Life over this masterwork of a sequel. Narratively Valve pioneered the very idea of telling an entire story through first-person, invented a spectacular interlocking physics model that's formed the basis for pretty much every action game since and showcased some awesomely unique weapons like the Gravity Gun to let you mash it all together. Will you ever get bored of lobbing a hacksaw blade at an alien's face through the power of displaced gravity alone? Thought not.