A very simple concept and gameplay loop that expands out into the bizarre and fantastic.
Honorable mention: Ronin.
Bullet time, effectively turn-based ninja combat. Simple, regularly autosaved "go until you die, then try something different" gameplay loop and just a helluva lot of fun.
Honorable mention: Valley.
Smooth and thrilling first-person mechanically-enhanced parkouring along the way to investigating the mysteries - both ancient and more recent - of a unique and very picturesque valley.