I don’t have hard numbers but from what I’ve read (and from my experience) if you’re managing the character it breaks down like:
Mage > dagger/dagger rogue > any other rogue > dual wield warrior >>>>>>>>>>>>> two handed warrior. Another thing to consider if you make your main character a warrior - you get several two-handed warriors in your party over the course of the game, but no dual-wielders.
Mage is just overpowered, they can do everything except pick locks and take hits… unless you pick arcane warrior and then they can do everything but pick locks. It’s hard to make a bad mage build as most of the skills are good to great, and if you know what you’re doing with bloodmage + arcane warrior you can probably solo the whole game on nightmare.
As for the rest, damage enhancement in the game (runes, weapon enchants, poisons) are per-hit, and two handed swings so much slower it just can’t compare. Daggers are faster then any other weapon, so even though they have lower damage the boost from damage adds is enough to compensate.
Dual wield wins on warrior for the same reason. The majority of the fights tend to be groups of enemies and dual-wield has most of the AoE attacks so it wins there too. The only thing two-handed has going for it is a knockdown on a short cooldown (pommel strike), the skill that makes you almost impossible to knock down, and better armor penetration (which does not matter for the vast majority of the game)
Archery is… meh. I was never impressed but I never built any of my characters around it. I suspect if you pump dex and use a shortbow it’s better then two-handed warrior but not as good as dual wield warrior.
As far as healing goes, a single mage with heal/regenerate (or whatever the Heal over time is called) will do for most of the game. Either manage them yourself, or load them up with lesser lyrium potions and set them to auto-drink when they get low on mana. Spirit healer gives you access to mass heal which is excellent, but is otherwise rather meh. If you don’t mind chugging down healing potions like crazy you don’t really need a healing mage at all though - you heal really fast after battles are over potions are pretty powerful and quick to refresh.
If you decide to go with your mage, some tips on spell lines: the mind blast line is excellent, sleep/waking nightmare line is nice, cone of cold is overpowered. Shock from the electric line is pretty nice but the rest of it sucks (stay away from chain lightning). The line ending with mana clash mostly sucks but mana clash is worth 4 points alone - huge aoe, instant cast, moderate cost and it will one-shot any non-boss mage in the game (who are, by far, the most dangerous enemies) and leave anything that survives with 0 mana.
I’ll put this final mage tip in spoiler tags because it’s really powerful and will make the game way too easy. Seriously, don’t read it unless you don’t care about losing most of the challenge from the game:
spell power + blizzard + lightning storm is a combo spell called storm of the century. It has a huge AoE and does MASSIVE electrical damage. You don’t need line of sight to cast it, so you can for example cast it on enemies behind a wall that can’t even see you yet. Just turn spellpower on, cast blizzard, then cast lightning storm right on top of it - it should turn into a gigantic electric spewing tornado that destroys everything if you did it right.
I haven’t messed with all the specializations, but stay away from reaver and shapeshifter. Spirit healer on mages is worth taking for mass heal alone if you don’t know what else to pick. Blood mage is great if you build the character around it specifically and obtain the blood mage boosting items but that’s probably best for a second playthrough after you’ve already unlocked it. Berzerker is decent for dual wield, meh for two-handed. Captain seems pretty nice, but I’ve not messed with it much.
As for stats, well. Warriors want STR all the way, if you go dual-wield get the min dex required to max out those skills (32 I think) and drop the rest in str. Mages want enough willpower to run whatever constant spells (maybe 30 at the most, prolly can get away with 20 - 25) you want on and still have some MP left, and the rest in magic. Rogues want the lethality trait, enough dex for the dual wield tree and enough str to use whatever armor you want, then pump cunning.
constitution only gives 5 HP per point so it’s mostly useless for everyone. Willpower also only gives 5 per point so skimp that as much as you can get away with. Cunning is mostly useless for everyone except lethality rogues. You need 30 cunning + all lock pick talents to open every lock in the game (not that chests ever have anything good in them), and if you want to take the speech talents on your main character whatever the minimum cunning required to get the last one is enough.
But really, don’t worry about it too much. Even a bad build with a poor choice of characters can finish the game on normal with enough health poultices (hint, some vendors sell infinite of the ingredients, so keep an eye out).
Anyway, sorry for the mountain of text, I hope it helps. Feel free to ask if you have more questions