I don't think it's too hard to make the case for Dark Souls.
It has some of the best combat in gaming. The environment is intricate and overall very well laid out. It has diverse leveling choices. There are storylines in-game and your actions can affect how characters interact with you and their place in the world. There are a lot of cool details built into the world itself, without requiring hours of cut-scene explanations. It's one of the few games where your progression of skill as a player has a direct and monumental impact on how successful you are in the game.
Those are all pretty accurate, I'm probably just miffed because it's taken me years to get anywhere in that game. I should really attempt it again, thanks for talking it up.
One great thing about Dark Souls is that it feels fair. It's brutal, but fair. When you die it's usually because you messed up.
It's frustrating when games don't play by their own rules, or they change the rules mid-game. Dark Souls is pretty consistent about what you can and can't do.