Dredge.

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 first-person mecahanically-enhanced parkouring along the way to investigating the mysteries - both ancient and more recent - of a unique and very picturesque valley.