What are the most unlikley possible custom and BG NPC party's possible? By this I mean what sort of combinations would never in no possible way ever happen. Here's one that I can think of.

Cavalier
Inquisitor
Undead Hunter
Bountyhunter}
Assasin }Any alingments
Swashbuckler}

I personally can never see a combination like this been formed unless there were no other choice, It's also very powerful in Melee I'd think.

Or another one like

Wizard Slayer
Valygar
Inquisitor
Bard
Sorcerer
Assasin

Diviner
Enchanter
Nalia
Edwin
Transmuter
Abjurer

Anyone care to try playing with that party?

Wizard Slayer
Conjurer
Diviner
Necromancer
Illusionist
Transmuter

Casey, do you mean unlikely in that someone would play it, or unlikely from a RP standpoint? Based on your post, I'm guessing the latter. :)

Generally, I think unlikely pairs would be:

Paladin + Thief (esp. Assassin)
Different kit Clerics (esp. Lathander & Talos)
Wizard Slayer/Inquisitor + Mage/Sorcerer
Evil Clerics + Paladins (esp. Undead Hunters)

For me, I guess it's really a question of which character classes have the most rigid code and/or beliefs... paladins, clerics and their divinely granted powers, and the fighter kits designed to kill a specific type of enemy would all fall into this category IMHO. Individuals, of course, can have their own issues--Valygar's hatred of mages for example.

If you're looking for the most unlikely party that someone wants to play, take your pick of six Beastmasters or the semi-finalists from Aldeth's least favorite NPC poll. :)

Well CamDawg they don't appear so weak that no one would wast their time on it and they are also some fairly interesting combos, so, given that, yes it's the latter :)

And Also Inquisitors only hate evil mages so thats one variable, and what about C/T? say if there was a Thief that decided to become a Cleric for Lathander or Helm, would they work with Paladins then and would it even happen? Theives can be important and have very useful skills for these guys but there are some thieves like the Assassin and any evil thief or a F/T that I don't think any Paladin would work with.

Edit: I have finally lost my IotW thankyou Tal :)

The only cleric thieves I know are cyrics chosen. Im sure there are other rouge gods, but ive never heard of a "good" one so to speak. And all the paladins in BG are good (ie no evil gods can have paladins in BG for some reason), so on a whole I dont believe that youll ever see a C/T and Paladin adventuring together in BG2, from a rp perspective.

To Rastor : I think that could be a playable party , at a certain level your characters will have all they need to beat the game. Think about those partys of Cowled wizards/enforcers , you can't beat them so easily can you?
Furthermore you can turn a mage into a good fighter (Tenser's Transformation) if need be , or just use the summons!

I think that would make a very powerfull party , especially at high level!

Except straight mages make holpless fighters with one attack per round or two with improved haste, and horrible HP and AC and the fact you can't use spells with Tensors activated make it a purely Fighter/Mage, Thief/Mage or Bard spell.