Dragon Age II TIERS
Tiers based on sheer/immediate usefulness, ability to dmg, etc.
God
Hawk - This is a given. Hawk is without a doubt the best character in the game, and can pretty much solo everything when properly built. Hawk attains customizations that would otherwise put other character traits to shame, it naturally says how truly diverse hawk can be. The Rogue Hawk is perhaps most dmging character in the game, which is even better sported with abilities that lower your threat/hostile percentage. You are basically killing everything while the CPU doesn’t even notice you, simply put. And, if it did, nothing would be able to touch you with high diversity/cunning/chameleon’s breath. The Magic Hawk is the best crowd controller in the game, and the best healer. The Warrior Hawk is perhaps Hawk’s weakest build out of the 3, but it doesn’t matter because it will be better than any other characters you’ll get.
Top
Merril - Merril’s dmg output is right up there with Hawk Mage, being the second most character you can get to cause high dmg. With the addition of gifts and personal accessories to boost her HP/Traits, she really is a force to be reckon with. She is a great crowd controller and has nice buffs thanks to the Entropy tree. Merril’s uses BM with other capabilities that help her when she’s low on Mana or Health, her only real down slide is not having access to Creation tree. With a Tank she really shines because the CPU won’t target her much at all, allowing her to destroy even the most powerful enemies in the game.
Mid-Top
Aveline - This is your tank of the game, nobody can do it better than Aveline. Some people think Ferris can but he’s more suited for action, a properly built Aveline is an almost necessity on nightmare. She has great HP, and can take dmg for your other characters who might be on the frail side. With items already boosting her high armor she can tank effectively while rogues/mages debuff foes alike. She doesn’t really work well with other warriors, so don’t pair her up with Carvar or Ferris because they will only disrupt what she brings to the table. As a tank, she also can be quite the dmg dealer. Again, it depends on the build.
Anders - Hate him or not, is your healer for most of the game. And on nightmare you’ll need all the healing you can get, trust me. He gains access to Heroic Aura, which grants some nice buffs. He can crowd control to a degree, but not better than Hawk/Merril. His Vengeance traits is put to shame by Hawks spirit healer customization, but it’s there if you need it. He’s usually prefer in a supporters role, so use him for what he’s meant for.
Varric - What I like about Varric is his weapon LVs up as he LVs up. This makes any weapon that Sebastain would get rather lousy, and he has good personal traits over Sebastain. Varric to me is the better archer, but Sebastain really isn’t that far behind. He does seem harder to hit than Sebastain, and with abilities to lower his threat/hostile percentage he can safely run around causing all sorts of havoc.
Mid
Bethany - Basically Anders, but with a little more offense. You can only use her for so long until she leaves the group, that’s her selling point. This pretty much drops her down, but not as much as Carvar since she’s the better for team builds. She can heal, crowd control, she basically has what Anders and Merril lack. She doesn’t have a personal customization, this hurts her even more.
Ferris - He’ll probably end up being the best Warrior you’ll recruit, but not better than Hawk by any means. Warriors in general are bad in this game, you can do so much more with Rogues/Mages. He’s not particularly good in harder modes or even when pared with Tank. In other words, he’s hard to build around. There are better characters in this game anyway, but when built properly he will get the job done in most cases. That much I can say about him 
Sebastain - He’s a good archer, and his personal tree makes him even better. He’s usable, but the argument still stands with me that Varric is better. For DLC character, he’s rather lacking TBH. If you don’t like what Varric brings to the table, use him (though I don’t see how you can not like Varric)
Mid-Low
Isabela - Here’s my problem with Isabela: if you are a Rogue Hawk I see absolutely no reason to use her at all. Unless, you get stuck in her personal quest and are forced to. If you are a Mage Hawk, you still won’t need her because there are better characters that take those slots she would otherwise fill. The only team I ever see anybody using her with is Hawk Warrior, she can do some goods things, but is more suited for easier modes. She’ll never reach the potential Rogue Hawk will amount too, might as well go with Rogue Hawk if you want a dual-blader.
Low
Carvar - Carvar is the weakest character in this game. He has no personal traits, leaves the party, and generally isn’t useful at all on harder modes. He’s a warrior and warriors aren’t good in this game, but this is especially so for Carvar because he’ll never amount to anything at all. Don’t play him or you’ll be wasting a slot in the party.