Demon's Souls, easily.
The first and the best souls game, one of the most important games from the 2000s spawning a whole sub-genre from itself.
Amazing atmosphere, amazing design, amazing lore, hard as hell right in the middle of the age of hand holding games, "wanna progress? cool, beat the whole level and the boss in the same go and you can progress", one of the funiest online mechanics ever with being able to post fake hints in everyone else's world and also being able to invade them or get invaded yourself.
It's just a freaking masterpiece, and the game I was sure was going to be 7 gen's best until the very until Naughty Dog decided otherwise.
Mad respect for this game, my first true platinum (had gotten Assassin's Creed 2 before just so my PSN account would lose its virginity because I didn't care about trophies yet), and also the game that made me be friend with one of the coolest guys that has ever joined this site, estebxx.