This one simply has it all, and it not only addictive, but also amazingly polished. I'm especially enamored by the creepy atmosphere, the best dungeon design I've ever seen (some of them are TRULY wicked!) and by the awesome physics-based combat engine, truly some of the best combat I've ever had in a game. Yes it hasn't got much of a story, but it wasn't meant to, it was meant to be a "show, don't tell" affair. There are just so many epic moments contained in it, and none of them are conveyed with a single word.
When I got my PS3 slim I discovered I couldn't transfer my 35 hours-long save from my launch PS3 with the borked Blu-Ray drive to my new unit because it was locked. I just started a new game and getting to the same point as I was before took about half the time, and it was an even greater experience than the first time.
I'm not sure it's my favorite game ever yet, I think I'd tend to be partial to something like Deus Ex or Tactics Ogre, but Demon's Souls really deserves the respect of any true gamer. Challenging, beautiful, haunting, intelligent, it's a work of art.
Sad that SCEI is almost guaranteed to never fund a sequel. Although it was made by a relatively small team at From and it has got legs, they probably lost money on it, and the game is plain too hardcore to have any sort of broad appeal.