This Day in Gaming - Dishonored
When you take inspiration from Assassin's Creed's stealth, turn it first person and add combat from the makers of Batman Arkham's combat flow, you have one hell of a gem.
For many FPS fans overlooked it at the time cause it was not adrenaline pumping pew-pews. However, it had a pacing like Bioshock and deserves more recognition than it has.