[Aerich]

"The most powerful party possible" is also difficult to pin down because of two things:

1) Stage of the game - a party of fighters to be dual classed to spellcasters ends up powerful, but may suffer at various stages of the game before its potential is realized. I've found a bard to be immensely powerful from lvl 11-20 or so, but a lvl 30 mage or cleric is significantly more powerful than a lvl 30 bard.

2) Playing style and personal preference - what's better? 2 clerics and 1 druid, or 2 druids and 1 cleric? See what I mean?

With those qualifications out of the way, I believe what best distinguishes a very strong party is balance. Multiple casters of arcane and divine spells, excellent melee skills, good archery, and enough thieving to get by.