The best characters in IWD 1 are multiclass.
A half elf ranger/cleric with favored enemy undead with roflstomp everything. If you don't use a shield, IWD gives you an extra attack per round. And you get to cast all the cleric spells and most of the druid spells too (I think in vanilla they automatically got all druid spells, but in HOW they only get the spells when the ranger level is high enough). You also get bonus spell slots from your ranger class. And you can specialize in weapons, which is the only advantage a pure ranger would have. It's an incredible combination. You have a character that has both excellent stealth AND turn undead. This is the most OP class in the game. No min-maxed party is complete without it.
Next go for fighter/thief. Make sure you specialize in long sword so you can backstab. Pure thief is boring and useless in 2nd edition.
A pure class cleric is meh, you're better off with a fighter/cleric, but a fighter cleric has absolutely no advantages compared to the ranger/cleric, it's basically just a crappy version. So you're better off with a fighter/druid, due to their powerful offensive spells and elemental transformations at higher levels.
So, basically, a min-maxed party should have:
Fighter/Druid
Ranger/Cleric
Fighter/Thief
If you want to have more magic power, or a fighter that can buff himself with strong mage spells, then add a Fighter/Mage too for extra arcane spellcasting ability. And if you have a fighter/mage that can cast all spells, you can safely make your primary mage a specialist to get an extra spell slot per level.