So I made my list based on how much easier the character makes the game on PotD (thats the true meaning of power imo). That includes ease of use, as well as availability of build defining weapons/talents and how powerful the build(s) are in all stages of the game. I try my hardest not to cheese and not to abuse choke points so I like melee heavy parties that protect one or two less defended ranged characters. I also don't like respeccing unless I am trying to fine tune a new build. I hate running back and forth and like to try and finish dungeons with 2 camping supplies + whatever is inside. Also I am assuming use of what I consider the strongest builds.
Wizard - best control best melee damage least squishy caster - mechanics bot
Priest - best support/healing, can be fairly sturdy
Druid - best ranged aoe and strong control. good mix between buffs, damage, and control
Barbarian - firebrand build is insane aoe melee dps and available way earlier than citzal's. Gets plenty tanky pretty quick as well, can also be a back-line control machine with tall grass
Fighter - intelligence builds are very strong. only viable full tank and can be really strong unkillable melee dps that is useful with a wide range of weapons. The knockdown tank build is somewhat weak until 7 but is an awesome party leader and ok as an early game meat-bag/knockdown bot.
Rogue - ranged build with borresaine. super early weapon availability, high damage and strong target lockdown. super powerful at dealing with the more annoying casters and enemies with high enough defenses to defend against aoe control. - also easy +3 mechanics
Paladin - really great abilities but outclassed by priest at all phases of game. no suberb builds except full support tank that is imo just a worse priest that can take more hits. I'd rather dump perception and pump resolve on my priest and make him a support only that can take a few hits. I like paladins though so I usually bring one anyway.
Monk - the best builds are really micro intensive. really strong but way too much effort to squeeze the most out of him.
Cipher - really strong damage but I find the abilities too annoying to use and he can't handle the front line like the wizard. too outclassed imo
Ranger - if the pet death didn't give the debuff I would like this class more. best single target ranged damage but the rogue is almost as good and gets free control as well.
Chanter - really useful late game with dragon thrashed but is too useless until then. master of cheese though