This is what I consider to be the most powerful powergamer party for IWD/HoW:
Paladin
Ranger
Druid dualled to fighter
Thief dualled to fighter
Fighter dualled to cleric
Fighter Dualled to mage
Bard
Paladin and Ranger have unique powers and priest/druid spells for backup cleric support. The bard is a must for instruments, bardsongs and backup mage support.
You don't need a full thief for IWD - dualling to a fighter lets you use the awesome F/T equipment out there (as well as the bonus of using all weapons, fighter backstabbing etc.). Max out traps and locks and stealth will take care of itself. The bard will pickpockets if you really need it.
Druid to fighter gives you the kick-A druid spells without the limits of a full druid (poor level advancement benefits) as you develop later as a fighter.
The fighter dualled to cleric gives you better HP and proficiency with weapons. Ditto for the mage - you also get to use whatever weapons you want (and mastery until you dual).
The Ranger could be substituted for a straight figher. The Druid/fighter could be a Ranger. Fighter/Cleric could also be a multi R/C (and the D/F changed to a Fighter ).
Mage could be Multi F/M too.
Phew!