For single target attacks, the Ice mage's Frost Ray does moderate damage, but thanks to the movement debuff, it often downs or kills targets instantly. Magic Missile from an arcane mage is another that does moderate damage, and tends to spread it over a lot of areas on the target, but the confuse effect can remove a target from the fight nicely.

For AoE damage, necromancers can send in a few undead to draw the enemy's attention, and drop fog of torment on them. That harms and debuffs the enemies, and heals the undead, and then they can start making corpses explode when people start dying, raising them as additional undead, or consuming the corpses for additional mana to keep casting. In a similar vein, blood mages who catch a clump of enemies with Rend, and then toss Ignite Blood, can wreck a raid. Fire mages are probably strong here, but I'm leery of setting the battlefield on fire, burning up both useful loot and recruits.

For tanks, I'd put death knights on top once they've got enough levels in. Shroud of Darkness just keeps improving their passive defences, and Life Steal keeps both the death knight, and any nearby friendlies, healing as the fight goes on. Monks have a flat reduction on incoming damage, so they might actually be the best option against swarms of low-damage enemies, like a lot of manhunter groups.

For utility, I'd give special mention to chaos mages, who can fill just about any gap in your colony's caster pool if you're patient enough with the rerolls, and wanderers, who can grow into very versatile mages, without occasionally giving the entire colony food poisoning like the chaos mages. Commanders running the Provisions aura can keep all of your pawn's gear in pristine condition. Geomancers with the Meteor spell can slowly seal off the edges of the map.

I'd agree with you on healers. Priests likely deserve to be the best, given that they get stuck being incapable of violence.