Baldurs gate, planescape torement, or arcanum?

Neither. Buy Sacrifice and play that first. Then Planescape.

Depends on what you're looking for, but I'd say overall I'd recommend both Arcanum and Planescape depending on the atmosphere you prefer.

I haven't played Torment. But I'd say Arcanum, then Torment, then BG. Reason: Arcanum is great, you will want to play it as soon as possible. There are also no loose ends or anything, once the game's finished, the story is too. Then Torment, because apparently it's kept in high regard and most people think it's the best RPG ever. I believe it has no cliffhangers either. And finally, BG, because in case you like it a lot, you can jump straight into BG 2 which, from what I've heard, continues almost immediately where BG left off. And I believe Baldur's Gates are the longest (especially together, naturally) and you can really spend a lot of time in them.

From what I've heard of others I would say:

Torment if you want a great story and characters (this I agree with having played it myself)
Baldur's Gate 1+2 if you want a balanced adventure (story, characters, combat, interaction etc)
Arcanum if you want an interesting skill system in a rare setting (half fantasty/half historic?)

which Baldur's Gate?

Baldur's Gate, because the other two games will probably feel so much better for you that playing BG after them feels like a chore.

Super important questions:
Are you familiar with AD&D2E?
Do you like dying a lot?
Do you enjoy games that just say "you are the chosen one, save the world!" without any explanation behind the over-used cliche?

If you have answered yes to all 3 of these questions: Play BG first.
If not, start with Arcanum or Planescape.

Planescape has a super unique story and some screwed up version of AD&D2E rules, I'll admit I've never been able to get in to the story, but the setting is awesome and it's one of the early games that really gives you real free choice on how you handle some things. It's also got an insane setting. On the flipside though, it's very low action.

Arcanum is one of my favoritest RPGs of all time, the story is semi-interesting, though it does have the "YOU ARE THE CHOSEN ONE!!!!" start.. except it comes from someone who seems crazy and is blathering about like a fool.. so it starts leaving you to decide what this bastid is going on about. It's not overly forgiving early on, but still moreso than BG, and it's got an awesome steampunk world. It has a classless system that has both magic and technology, and lets you craft nifty things.
Just a fair warning though, it's also very poorly balanced and the rights builds are outright broken late game.

BG, on the otherhand, is AD&D2E rules- which are garbage for a video game. Plain and simple, 'by the book' they just SUCK for video games. D&D is designed to have one or two combats a day, not every time you turn around there's a fight.. which is what video game players expect.. yet they don't stop and revamp the D&D rules to suit a video game environment better..
If you can overlook that irritation, it has the most cliche start ever. You're an orphan, being looked after by this uber dude.. then something happens and it's revealed that 'you are the chosen one'.. but he doesn't get a chance to explain it to you.
The only remotely redeeming quality of BG (to me) is the cast of NPCs that can join your party.. they did a great job of capturing the feel of a real D&D party.. AKA a disjointed group of dimwits who are more interested in antics and fuckery.. which is fine in a D&D game if you have a party and DM that are cool with it.. but not in a video game.

Take that how you will, with the knowledge that I have tried to get in to BG many times, and every time I have been bored to tears by it, and .. just absolutely do not enjoy any aspect of it at all.
(and before "well you just don't like oldschool RPGs" I love Fallout, ToEE, IWD, and I want to like Planescape but the story just doesn't grab me...)

There was a thread with a similar question here:
http://www.gog.com/en/forum/general/for_the_next_game_to_get_on_gog_which_one_of_these_three

The consensus seems to be Arcanum first, and never ever play Sacrifice at all :-D

Just kidding, Licurg!

Arcanum

I'd say go for Arcanum first.
You can either play any of those 3 games, doesn't matter - really.
The fact that I'd go for Arcanum first is because Arcanum is kinda like the combination of both Baldur's Gate and Planescape - Torment.
Beside, Arcanum doesn't belong to any prequel or sequel unlike Baldur's Gate series. So, it's better for you to go for Arcanum first. It''s a damn brilliant game imho.

Planescape - of those three, it has the strongest story by far. Gameplay is vestigial, but most of your time is spent talking anyway.

If you are going for any Baldur's Gate, I recommend BG2, because that is epic and pretty well rounded. Arcanum tries to be original, but fails in execution in my opinion.

PS:T is the best RPG story ever told.

Arcanum is the best RPG setting ever created.

BG is the most mainstream RPG ever sold.

I know what I wouldn't pick ;-).