Portal. Originally designed to be a cheap spin off of the half life series, the game took on a life of its own, becoming something fairly unique, remarkable, and, overall, extremely enjoyable. It went further than the company that put the money into it likely ever dreamed; today, not cake is safe from being called a lie. I've had more fun playing other games, but none have taken on a life of their own in quite the same way; thus, I have to consider Portal the 'best'.