IMPORTANT!

And we're back but important thing before we start, let's keep this spoiler free or at least spoiler tagged!

For me it is:

Penumbra: Overture - PC

Brutal and unexpected, bit of an emotional sucker punch. :s

Red, my mad and only friend in the darkness. I discovered that Red had essentially grown up in the darkness of the mines, praying for death but was unable to carry out his final wish due to powers unknown. He gets into and setup an incinerator such that I can't release him from it, while holding a key that I needed. The only way to get out and continue my escape is to turn it on, burning him alive. I searched that room for another way but found nothing. His screams of pain filled me with an incredible sense of loss, sadness and guilt like nothing I've felt in any other game. Genius.