Party balance plays an important role in the IE series of games (BG, BGII, IWD, IWDII). Here are some basics of party formation and management for a 6-person party:
1. 2 (two) of the melee classes selected from the Fighter, Ranger, Paldin, Barbarian & Monk. Each have strengths and weaknesses.
Fighters are the strongest melee specialist in the game. They have the most feats of the melee specialists, giving them a tremendous selection of offensive/defensive feat selection. You can range from two-handed weapon tanks, to dual-weilding tanks to ranged weapon dexterity fighters. You can pick up many of the advantages of Rangers/Paladins by taking 3 or 4 levels of cleric.
Paladins fight nearly as well as Fighters, but are more limited because they trade feats for a limited selection of spells. Those spells are nice, but you get fewer feats. Generally, this ends up with you specializing in one weapon or giving up quality defensive feats, such as Iron Will. That's nice if you picked the right weapon when compared to the game's balance, but if you didn't...
Rangers get the dual-wield feat for free, but the offensive increase does not merit the defensive decrease; otherwise they're like Paladins.
Barbarians do not fight as well as fighters (once again, it's a "feat" problem) unless on "rage," and that ends, leaving the Barbarian weakened. Barbarians work better if dual-classed with rogue (Ghostwise Halflings or Half-orcs).
Monks do very well in man-on-man situations and are unbalanced toward PC favor in these situations. However, the opposite is true in crowds (can't use dexterity bonuses from flanking/rear attacks); monks are also weak in HP.
2. You need a (one) divine caster.
Good clerics have advantages in IWD:II. It's the nature of the campaign. A neutral cleric (Mask) can be helpful, especially with a rogue multi-class. Evil clerics get the short-end of the stick. You just don't get that many opportunities to take advantage of them.
A druid is a viable substitute for a cleric. Be warned, they don't convert spells to healing spells. This limits the Druid in flexibility.
3. You need an expert treasure finder.
If you go with Druid, I'd take a bard. If you go with rogue, I'd multi-class it with either cleric (mask) or wizard.
4. You need an arcane spell caster.
Wizard, hands down. Flexibility in magic is required. Make sure you can "divine" (identify) in your party if you're going to take a specialty school that prohibits divination. It's a darn weak spell school in the game, but w/o identify, you'll spend 1,000's. AND there are plenty of "cursed" items that you don't want to equip.
Sorcerers work, but *this* game follow the rules. You pick a bad spell, too bad, it's with you forever. Also, Sorcerers are like top-fuel dragsters - real fast, don't turn very well.
5. With your sixth character, you take whatever strikes your fancy. I've done tons of weird things. Bard/Rogue/Wizard hybrids. Fighter/Mages. Cleric/Druids. Ranger/Druids. Wizard/Monks. Sorcerer/Monks. Not all have worked well.
