Boy, Dark Souls. Hardest game (despite using a guide AND cheesing the hell out of a few major bosses) that I've played by a long mile. However, as the 5 stars indicate, this sense of difficulty lent to a gameplay experience unlike any other, thanks to the dynamic combat and focus on exploration being a key component in winning the day. While I feel like I barely know anything more about the game's setting and lore than before I even started playing it, I imagine that's what the sequels are for.