Most recently, Mark of the Ninja. The game really lets you play around with the mechanics and rewards good timing and creative use of the environment. Sure, you can just stab two dudes, one at a time, but you could also pounce on one from above, string his body up a tree, then lure his friend to it, at which point he freaks out and runs across a grate, at which point you stab him from below and drag his body down. Don't get me started about the hillarity that is dropping a dead body in the middle of a group of guards and seeing them all freak the fuck out and shoot each other dead.
The game encourages clever play and trying out new things with challenges on each level. These are entirely optional, but reward points that are used to upgrade your moves and gear. Also, fancier methods of taking out enemies reward more points, which in turn also rewards more upgrade points. As a result, you'll be doing your best to do all the convoluted stuff instead of just plain stabbing people. The fact that the kill sequences are wicked cool doesn't hurt.