Gladiator is the tank. You can armour him very heavily, or he can do huge damage when armed with a spear/sword combo. Gladiator is pretty good, but the real butcher is the Assassin. Get him tooled up with Black's Heart and another good dagger (I'm using Integris at present) as melée weapons, and Trolleri as a throwing dagger, and he does horrendous damage. When you push his blade skill way up, he has so many attacks that he does damage consistently - you're less at the mercy of bad dice rolls.
Priest has the advantage that apart from the Ranger, only he can use the best bow in the game, Everstrike, albeit only at a normal level. He has massive healing powers and some good combat spells, including access to Poison Cloud at a moderate level. When armed with Spencer's Obedience, he also does a fair amount of combat damage. Lastly, you cannot gain the XP for the priest promo quest without actually promoting a character to that rank.
Druid is probably the best compromise between a fighter and a magic user. He can GM the Unarmed and Dodge skills, and if you get his Unarmed skills up beyond the GM level, he gets an extra attack for every 3 extra skill levels, which makes him very effective in melée. He also has a bonny range of spells, although these don't include Poison Cloud. Biggest weakness is liable to be ranged attacks, because you're liable to want to give the other artifact bow (Kraftig Bage) to somebody else, and he doesn't have good inherent skills.
Lich and Mage are very similar, but IMO Lich is slightly better. The Dark magic spells are more attuned towards combat damage, and he has access to Pain Reflection, which the Mage lacks. His top spell (Souldrinker) is better than the Mage's Eye of the Storm, and he can master the Thrown Weapon skill (arm him with the throwing axe Tillhygge) whereas the Mage can only gain expert in it. Finally, like the priest, you cannot gain the XP for the lich promo quest without actually promoting a character to that rank.
Crusader and Ranger are both IMO a waste of space - use these only if you want to set yourself an extra challenge. A Ranger's only advantage over a lich in melée skills is his ability to use a wider range of weapons. Neither can get above expert in anything. OK, he's the only one who can GM in Bow and he can GM Dodge, but that's just about it; his magic skills are picayune. The druid, with GM in Unarmed and Dodge, is a far better melée fighter, and his magic skills are incomparably superior. The Paladin is better. At least he can GM armour, Blade & Cudgel, but his fighting ability is still greatly inferior to the Assassin and Gladiator. Also, his magic skills are simply ineffective, particularly later in the game when your high-level party meets higher-level monsters. You just try healing a badly damaged fully developed character with a Paladin's spells; it'll take forever, and use up most if not all of his mana - the Lich is actually a better healer than the Paladin.