I'll pile on the Dishonored love thread: the mechanics and level design are world class, well worth your time in themselves. But the amazing parts are the stories and storytelling. You get to see both sides of every argument, and if you care to go through it again making different choices you'll experience the same stories but with a meaningfully different connotations.

That's not even getting into how the extensions/stand alone DLCs complement the main storylines.

TL;dr: If you've got 30-40h go crawling around Dishonored's universe a couple times. (Go Arkane studios!)