Let's go for my personal class top :
Super God Tier :
1 - Priest : the answer to all your problems. You can't replace priest, except with ton of scrolls, and you still won't get Crown of the Faithful or Devotion of the Faithful.
Best buffer, best healer, very nice AoE damages only a bit inferior to wizards and druids.
Priest has mediocre CC for a caster, but that's about it.
2 - Wizard : Most of the best CC, crazy AoE DPS, especially with alacrity.
As powerful as Priest, maybe even more, just a bit easier to replace.
God Tier :
3 - Druid : AoE on par with Wizard.
Can bash everything in melee on basic encounters, which makes him very suitable in balancing your party.
Moonwell is crazy as Buff/Heal.
But his strongest card are Storm spells, especially relentless storm. Relentless storm makes crowd fight so much easier, especially against all the trashes that likes living in dragon's lair...
"I really like to have these guys in my party" tier : These guys don't have the raw power of casters, but makes party life a lot easier.
4 - Ranger
Best martial DPS overall.
Pet does very good damages and ends with very nice DR that makes it decent as offtank.
Twin arrows and, to a lesser extent, driven flight makes ranger's distance figthting really above anyone else.
He can snipes casters, and do reliable damage against everything else.
Icing on the cake : Crowd controls. Binding roots and Stunning Shots are 2 excellent sources of Single target CC.
Did you already face binding roots ? Then you know how annoying it is to have your melee guys Stuck for 30s. 30s "half-hard" CC as fast cast... and it is just 1 relatively minor card in the ranger's deck...
5 - Paladin
Best single target heals, sturdy against about every spells and attacks.
Zealous endurance will make early game far more stressful.
And immolation because AoE DPS is cool.
The ideal frontliner that buff everybody, saves life and does AoE damages...
6 Monk
Monk can do about everything : hold the frontline, deals very good DPS (with some AoE), on-demand CC and even powerful summons.
You just have to manage wounds accordingly. Which is not always easy.
That's the limit to his potential. The good point is that he has the most potential when it is the most needed.
7 - Cipher
You want a mix of martial DPS and caster ? Here comes the cipher.
His single target CC are probably the best.
Are you an archer ? Aren't those guys running to you annoying ? Cast Puppetmaster and now they are yours for 20-30s.
Slight weakness against boss cause focus generation will be a bt more limited...
Okay Tier : Not as much key members as the ones above, they have less potential in my opinion. But there is currently no bad class IMHO.
8 - Chanter
Dragon slashed hard, but Dragon was a bit one poney trick.
Still powerful enough to justify the class.
And in case Dragon won't be efficient, you can just change your trick.
Invocations are rare, but some are very powerful.
Ah, and chanter can hold the frontline very easily, which worths noticing.
9 Fighter
Fighter may not be the most powerful class but they are very reliable.
He hits as hard as Cipher, and hold the front as no other, as they have a couple of "immortality tricks" (triggered immunity and Unbroken).
Charge really added decent AoE potential.
10 Barbarian
Rather weak abilities. But carnage is godly. Barbarian will probably end up with the most damages.
Surprisingly, they are very nice offtank with their endurance, health and instant cast self-heal. In most cases, they will hold your flank wihout any problem, especially with a draining Weapon (who said Tidefall ?)
Heart of fury is probably the most damaging AoE of the game by the way, even if slightly situational.
Their downside is that they have close to zero utility when facing single targets. Which happens.
11 Rogue
Single Target damages. And that's about it.
Current rogue's problem is that they are in my opinion inferior to Ranger in this role.
Twin Arrows outdamages Rogues ranged capabilities, and combining Ranger and pet attacks will come at least very close to rogue in total DPS (rogue can achieve zero recovery with sky-high Dexterity, lashes, crazy and bonus damages, etc..., so they are probably above... but Ranger can have 2 arrows and a pet, which means 3 attacks per swing as multiplicator...).
But Ranger, as mentionned above, have other cards in his play, like Double Endurance Pools and infinite Single Target CC, and, well, ranged superiority.
Let's face it, the pet itself is at least as sturdy as the rogue, so there can't be too much arguing about "yes but pet is too fragile". Yes, but Rogue too, and Rogue does not have around 25 DR...
Probably the most convenient skill monkey, but that's about their only use apart Single target DPS.