Journey.
Not part of the story but kind of a spoiler, so look away if you've yet to play the game but want to...
... reaching the top of the mountain and having this player who I'd been on this epic 2 hour journey with draw a heart in the sand as we said our goodbyes. Bloody hell. It was an emotional sucker punch that I genuinely wasn't expecting and it really upset me, but in a good way. I found the guy's name afterwards (it was in fact, a girl) and sent her a message to say thanks.