Definitely Planescape. Since you get second one for free, I'd recommend you also get Baldur's Gate 2 (it's superior to the first one). Or the first one, if you care for the continuity.

I'd go Planescape, BG, BG2 and Neverwinter.
The first 3 are a must while the last one makes up in content what it may lack in quality.

I'll second this. NWN may not have the best original campaign, but you could literally play that game forever with all of the extra downloadable fan campaigns.

I personally don't think there's much point to playing BG and BG2 separately, so don't get one or the other. I'd say Planescape plus Neverwinter, and then BG1+2 if you decide to go for four games (which you totally should).

Buy Planescape & NWN (& get free TOEE).

Then buy BG1 & 2 as a gift.
GIft it to your own email.
Redeem gift yourself.

You now have 5 AWESOME games & your gift code still has TOEE on it for someone who hasn't been able to buy any of the games themselves....

Thirded! And the purchase order listed here makes the most sense as well.
BG1 + 2 are to this day my benchmarks for RPGs. NWN is my benchmark for customisability.

Planetscape, BG1 + BG2, IWD (just to be different) :)

I concur with your choice :P

I vote..Planscape.

Three possible combinations:

Planescape and Neverwinter Nights
BG1 and BG2
Icewind Dale and Icewind Dale 2

BG2 is better than BG1 but it's best to play them at the same time so you get a full grasp of the story, taking your character from a lowly student to an ascending deity is a unique game experience. Planescape is all about the story and very strong in all areas except combat, also the graphics are outdated, and NVN is just a massive game that's got a lot of character development. Icewind 1&2 are tactical combat games, decent story but no main character, it's focus is on party development and kicking ass.

If you do chose both BGs make sure you download TuTu, it updates some of BG1's gameplay and interface to be more like BG2, it makes the transition easier and also makes the game a lot more playable. You need both BG's for this to work though.
http://www.pocketplane.net/mambo/index.php?option=content&task=blogcategory&id=143&Itemid=98

Also don't forget if say you only settle on 3 you'll still get that third one half-off.

I believe that only applies if it's the last one you don't own.

Oh, yeah, you're right. Is it just me or did GOG purposefully make this sale confusing?

It looks like they had a brainstorming meeting to decide on the type of promo to do, then decided to use all the ideas at once!
I might take a bit of work to get your head round, but with this much awesomeness at this price, I'm not going to hold it against them....

You can definitely pass on Demon Stone and Dragonshard - They should have been $5.99 to begin with and are not on par with the rest. If I were you I would go with BG, IWD, NWN, and Planescape. You can always pick up BG2 and IWD2 in the future if you like the first ones (which I am sure you will).