Sid Meier's Civilization IV

Looking deep inside the code of Civ IV would be the gaining equivalent of climbing inside the Total Perspective Vortex from The Hitchhiker's Guide - so vast, complex and limitless in potential that your mind would be mulched instantly. Thankfully then, Sid Meier saw fit to nestle a beauteous interface over his creation - letting it feel like you'd got raw history ticking away beneath it all, but keeping it manageable and non-terrifying. From the very first selection of a decent place to settle to the point at which you first dropped the nuke (and all the pointy-sword squabbling in between), Civilization was so captivating that bedtime was always pushed into the wee small hours.

You couldn't help but give human qualities to your Al opponents, analysing their movements and trying to see through their meaningless platitudes and offers of bananas in excliange for plutonium. There was just no other game that provided such feelings of glory or impending doom - all through one simple 'end turn' key-press to boot. And with Civ IV, well that's the iteration that just nailed everything: multiplayer, looks, religion, music, modding, engine, Leonard Nimoy... Everything.