I reckon the two strongest parties in MM9 are (1) assassin gladiator priest lich, and (2) druid priest mage lich.

The first is a good all-round party with no weak links at any point, but lacks a bit in firepower in the final stages.

The second is difficult to keep in one piece in the early stages, but becomes horrendously powerful later. It's only weakness is in ranged physical attacks, where the lich is the most effective. Although the priest can use Everstrike, the druid and mage are not very good. The druid is far and away the game's best fighter/magic user, very effective in melée with GM in Unarmed & Dodge, and with a good range of spells. All three of the others can use Poison Cloud, which is a murderously effective combat spell. That party will take out the dragon with some ease.

I think ranger and paladin are definitely weak sisters in this game.