Dark Souls, I've been a gamer for a my whole life but I always enjoyed playing games on a lower difficulty since I was more interested in story and scenery than just upping the time it takes to get through encounters (legendary in halo for example, all it does is make the enemy shoot faster, harder, and gives them more health, not really changing tactics, just elongating them.) I actually had heard of demon souls but dismissed it because I didn't have a ps3 and the whole "hard game" didn't appeal to me from experiences with other games. Once I watched a lore video for dark souls though I picked it up on steam and it was fantastic. I was put into a world where I had no idea what was going on, characters didn't care to help me because they have their own agendas instead of just being an information feeder. Taking down bosses, with just a "victory" banner coming up was just so satisfying for me, normally bosses all fall in cinematic cutscenes, but not in dark souls, they just die, vaporize, and then silence.
They designed the game with the idea of overcoming challenge, everything from the depressed atmosphere to unhelpful NPCs, it was all just to reinforce the feeling of accomplishment you get when you get to a new area. I remember the first time I found a shortcut from the first bonfire in undead burg that goes up to the drake bridge, it was just a little victory, but it felt so nice. And throughout it all you'd still have player interactions from messages, invadings, summonings. So despite the fact that you are in a dark dreary world fighting for whatever reason you want, there is always reminders that you're not alone.