The strongest party I have made was in my DU playthrough, it was
DU 5 lvl shadow monk + 5 lvl rogue assassin + 2 lvl fighter, dual wielded, lot of invisibility + DU's invisibility cape, basically invisible every turn
Minthara vengeance paladin, no comments there it's a paladin
Laezel master of battle fighter - no comments needed as well
Shadowheart domain of life cleric with enjoyable Dark Justiciar skills