Perhaps the prototypical walking simulator, Dear Esther is a source of some division among the RPS Hivemind - John's not at all taken with it - but I often catch my mind's eye returning to its maudlin Hebridean coast.
Dear Esther is the essence of loneliness and regret, and pins by turns evocative and startling sights onto those universal emotions, augmenting them and also validating them with a vision of pretty purgatory. When I feel low, I long to retreat to rural solitude, rather than a poky suburban bedroom. Dear Esther makes misery noble rather than wretched, weaving music and environments into far more affecting an experience than any description could capture.
Not for everyone, I realise, but as an example of games trying Something Else, I think Dear Esther is hard to argue against.