Assassin’s Creed

Developer: Ubisoft Montreal
Publisher: Ubisoft
Platform: Xbox 360, PlayStation 3
This is wide-open in structure and graphically breathtaking (not only for the fluid animations of ltair, but also for the massive crowds that move and interact realistically). As an assassin in 1191 AD, you can accomplish your objectives any way you see fit, including scaling walls, sneaking through crowds, and utilizing medieval equipment. The world responds to your actions believably (for example, climbing a wall in public will cause alarm, which will have repercussions for your mission).