Neverwinter Nights 2. However, there are a few problems.

1. The reason I'm recommending NWN2 at all is Mask of the Betrayer, the first expansion pack. The NWN2 main campaign in good, but not great.
2. MotB is set in a time less than 24 hours after the ending of the main campaign (EDIT: This is never explicitly stated in-game IIRC, it's just speculation). It's not strictly necessary to play the main campaign to understand what's going on in MotB - you get a fresh roster of characters, and the locations are mostly different - but I would recommend doing so anyway.
3. As ever, the D&D rules can be confounding to work with. NWN2 uses 3.5e rules which are much more complex than Torment's adaptation of AD&D.
4. This problem is further exacerbated by the fact that MotB begins at level 18, or possibly higher (19 or 20) if you import a character from the main campaign. You're likely to hit level 38 or so by the end of MotB, by which stage the number of rules simultaneously affecting every single character can get ridiculously high.
5. Like the main campaign, MotB assumes that you have a reasonably good understanding of the Forgotten Realms setting. You can finish the game even if you know absolutely nothing of where you are or what you're doing, but you'll probably be very confused throughout most of the experience.

On the plus side, Mask of the Betrayer is generally held in very high regard for its story and writing, which presumably should be right up your alley.

I'm in the same boat as OP; I loved Torment, but can't stand the BG series. I managed to struggle through IWD (which I would not recommend), but NWN and IWD2 lost me out of the gate with their implementation of 3.5 rules. Character creation took forever as I tried to make sure that I wasn't making decisions that would screw myself out of feats later on, and in NWN the lack of agency I had over the actions of my avatar kept me from getting absorbed; I'll gladly take handling each member of the party if it means that I will have more of an impact on combat than telling my Paladin to autoattack once and watching as they work their way through a horde of goblins.

How is Arcanum then?

Torment is one of those very rare games that are hard to find peers for. The quality of its writing is exceptional and I've never seen anything match it. PST was written for a contemplative, adult audience whereas most RPGs seem to cater thoughtlessly the hack-n-slash crowd.

That said, both NWN2: Mask of the Betrayer and the not-yet-mentioned Star Wars: Knights of the Old Republic II share some of the development and writing talent of PST so they're definitely worth at least checking out. They don't cost very much these either.

Both games are much more combat-focused than PST, but their writing is still strong so if you are willing to spend a bit of time learning the combat mechanics, it might well pay off.

For NWN2, melee classes are severely gimped in 3.5e unless you go for a min-maxed fighter with exactly the right feats... it's a very unforgiving character creation process. To remedy that you can either go for somebody else's melee character build template (Google them) or just go with another class. Without giving too much away, Spirit Shamans will have a MUCH easier time and let you enjoy the story more without constantly dying.

For KotoR2, the 3.5e rules are still there, but the melee classes are much more viable because they're... well, fucking Jedi. The rules are also somewhat more invisible and the game just isn't as hard as NWN2, allowing you to focus more on the storyline. KotoR2 didn't see much mainstream critical success, similar to PST, but I think the quality of its writing shines in its nuanced treatment of morality... it is the only Star Wars game (that I know of) that discusses morality in more than the simplistic binary treatment of the light and dark sides of the Force. Even if you're not a Star Wars fan, seeing it as a novel about the will to power and its effects on the psyche still makes for an interesting story.

Arcanum, on the other hand, I found exceptionally boring. The dialogue seemed simplistic and I tried so many times to get into it, but couldn't shake the feeling that the story lacked any sort of "hook" or theme, at least in the first few hours of play. Maybe it gets better later on, but I never was able to stick around long enough to find out.

And in less detail, other story-based RPGs with decent writing you might look into:
Final Fantasy 7 for its emotion
The original Deus Ex (FPS too) for its foreshadowing of the modern world's security paranoia
Dragon Age: Origins for its character development and interactions
Mass Effect for its great acting

And if you don't necessarily need the RPG element, a lot of the old LucasArts and Sierra adventure games are superbly written, and the genre is just recently coming back to life with a lot of smaller publishers picking up where the old guard left off.

Good luck... hope you find something! I'm desperately waiting for the new Torment and Eternity to arrive as well.

Go for it. As Unworthy mentions it's actually quite slow in the beginning so avoid playing it in short sessions (at least in the beginning) so you quickly as possible get out in the world and get a sense of what you can do and what the main quest is about.

Also, check the forums about High Quality music, map, some bug fix update and widescreen in case you're interested in that.

And before you even purchase Arcanum, find some guide that explains the mechanics because it's quite easy to mess up your character.

Thanks for your suggestions,
I'll try to buy DAO when I can.
Mass Effect... I'll try to get the explosion ending out of spite when I complete the third one.
FF7 I played long time ago, and I feel FF6 is a better story when I play through it.

Thanks for your suggestion.