SI is pretty great, you should definitely do it.

Don't have the game right now (was pirating it back then). Will consider getting once I clear the backlog a bit.

SI and KotN both are excellent. In terms of atmosphere, I wish all of Oblivion had been as cool as SI. The fighters guild is totally forgettable, but you ought to finish the main quest once. Some of the later areas (like Paradise) are pretty neat, but the story itself is kinda meh. I really wish they'd given Sean Bean more lines.

As mentioned many times, The Witcher is a truly great tale. The Witcher 2 is even better. It deals with philosophical issues without being hamfisted and has the quaint feel of a classic RPG. Even if you think certain characters are ridiculous or cliche, you still find them interesting, and they speak much truer to real conversations than most exposition dumping "worlds" (a growing trend, it seems). Things feel real and while the story does get derailed, there is a brilliant pace to it which I haven't seen matched by a modern title.

I absolutely hate that people insist that Mass Effect has a good story because there is so much background to it. A story of a game should be about what happened in the game. The interactive element is there for a reason. What you play out in the first to games (I'm not invested at all to play the third) is so confusing and yet surprisingly simple that you'll wonder what in the flying hell the story even was. You might remember events, but there's hardly anything to tie together a cohesive story.

KotOR, also mentioned, is truly a game worth playing for story alone. It's not like Mass Effect where the choices is all anyone really cares about (though the choices exist) but rather a true universe of things to discover. It helps that it has the Star Wars name, of course, and it truly feels like a grown up Star Wars adventure (far more than Episode 3 even attempted). There's portions which are non-linear with scripted events which keep the plot intact and even the villains you become very invested in. It's truly a narrative driven game that exceeds its roots.

I'd also say the Ultima series from Utlima 4 to Ultima 7 Part 2. These games are about true moral conflict and a real expansive world to explore. There is perhaps no series of RPGs that went deeper into forming the world and its goals better than Ultima. The virtue system alone makes it a game worth studying into, but what it does with those virtues is what makes the quests of these games truly unique. Particularly Ultima 5, which is absolutely brilliant in its implementation of mechanics and the overall game world. Not a story to miss out on.

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.

Oh yeah, Psychonauts is good if you want something more light-hearted.

The campaigns from "Battle for Wesnoth" are mostly well written, and it's a freeware game too.