I picked Dark Souls although it had many flaws.
Playing through Blighttown with me and my 2 best online friends was an experience that few games can rival, and it really took me back to our old L4D tourney days. Knowing that you only get 1 life, and everything in the place was trying it's best to kill you was crazy. Working together we managed to get from the entrance ladder to the bossfight, that's using only the 5 starting flasks and no bonfires.
I mean sure the COOP is brokenly bad in that it takes forever to get friends to join, and you can't use bonfires with companions, but in a way that made the entire experience an adrenaline rush. Knowing that every person was just seconds away from death. Stumbling around in the poison mists trying to find the correct path and refusing to be the guy who dies and lets his friends down. I can happily say that we made it to the bossight together, and while not everyone made it out alive by god we did what few others could do.
When I play other RPG games, or really almost any game, the difficulty is adjusted by multiplying the AI's HP and damage by a number. In Dark Souls there is no difficulty slider, because difficulty is set by good AI and intelligent traps/puzzles. Not many other games can say that.