The user-created Prophet module series for Neverwinter Nights. It's not Torment, but when it comes to gripping storylines beyond the average RPG clichés, it would be my second choice, despite not being a commercial game campaign. I haven't played many games with a more interesting and involving story, for me it's like a page turner in the form of a video game, only that I'm rarely interested in actual page turners in book form. I wish they were more like the Prophet series. ;)
Gothic and Arx Fatalis have okay storylines; they're both great, immersive games with interesting and convincing settings but their actual plots aren't all that original, just a little bit better than the average generic fantasy RPG. If that's enough for you, you could also try Vampire The Masquerade: Redemption. I liked all three and enjoyed their stories, but they're nowhere close to Torment.
From what I've seen of Giants: Citizen Kabuto, I'd say it won't win any prizes for story either. It's equally silly as that of Psychonauts, but I've found the latter much more original and consistent. So if you don't mind a funny story with lots of silliness in it, but full of surprises and creativity, I can also recommend Psychonauts.
Also, the indie "adventure" To The Moon, if you put story before gameplay; or Gemini Rue.
Post edited June 24, 2012 by Leroux