SS: Psionicist
They're like Wizards, but can keep casting their best and latest "spells" (Powers) repeatedly without concern for spell slots, and most Powers scale can scale down for efficiency or scale up for greater effect. They don't have to worry about being Silenced, and they don't have to worry so much about being Grappled, Paralyzed, or Encased as they can keep casting. For area damage, they're unmatched overall and will probably outperform even a Red Wizard due to being able to spam Energy Ball nonstop while always choosing the best type of energy and the best defense to attack (Reflex vs Fort). Their control, debuffs, and save or suck Powers are just as good though.
S: Wizard, Cleric
Wizards get so many spells, so many unique spells, so many great buff/debuffs, and so many unique Feats that you probably want one in your group regardless of your group's strategy.
I don't usually rank Clerics so high in D&D/Pathfinder type games because high Initiative arcane offense is usually best. But in KotC2, there are lots of enemy casters and you probably can't prevent them all from casting, so the ability to prevent/remove bad things is more valuable. And there are improvements to Cleric spells like Sound Burst and Holy Smite that make them great on offense/debuffs too.
Wizard Attunements are all good so long as you make use of them in some way, but I think Green generally wins out for Move Action Haste. Similarly, while many Cleric Domains can be great so long as you make use of them, Mysticism would be my top pick for Move Action spells (and increased effect on them).
A+: Druid
They're efficient, versatile and they have nice summons. But they're ranked slightly lower than the other pure casters because they're missing too many good things: Accelerate, Break Enchantment, Mass Heal, True Rez, etc. Also, they're limited in small spaces where big area spells would hit the group and where large summons may not even fit. They do get several outdoor late game fights to shine in, though.
I'd put Earth above the other Spheres, but not by much. They're all good.
A: Fighter, Bard, Mage Knight, Rogue
Fighters get Wade In and tons of unique Feats, but fall off a bit in the late game.
Bard Songs are always useful and the +DC song scales particularly well into the late game because it becomes increasingly difficult to debuff enemy saves. Bard Spells can fill some gaps in your group, but Silence is probably going to be useful if nothing else.
Mage Knights can be extremely tanky, very good with Touch spells, and also very good with area damage. Consistently solid class all game.
I know ranking the Rogue here may be a little controversial, but mine are easily on par with Fighter kills in the early/early-mid game and effectively turn into casters for the late game because of the piles of high level Wizard scrolls the game gives you. The only time they struggle a little is in the mid game against certain enemies, but if you make good use of your Rogue spells and scrolls, they're still not bad. They're on par with Fighters (to me) because they tend to be tankier, more mobile, and do better in the late game.
A-: Psychic Healer, Sorcerer, Warlock
All these caster hybrids are weighed down by the fact that they're Multi-Ability Dependent. Add to that the loss of unique Psionicist/Wizard/Cleric Feats and slower spell/Power progression and you're almost always better off as a pure caster. They're still generally good by using half their spells/Powers for offense and half for defense/support.
B+: Monk, Psychic Warrior, Bishop
Monks are fine, but Fighters are usually better. That pretty much sums it up.
Psychic Warriors are great Grapplers and actually get some area damage later, too. But they're rather limited on Power Points and often can't justify taking time to cast slower buffs in combat. It's a fun class, but they're not as effective as many others.
Bishop is a mixed bag. The combinations of Domain Powers can make for unique/versatile characters, but usually you're just better off with a Cleric instead. They can be rather strong earlier in the game, but they're going to fall off compared to a Cleric later.
B: Barbarian, Death Knight, Storm Warrior, Champion
Considering how much the guide repeatedly focuses on Grappling, I'm surprised it ranks Barbarians last because they're great for it. The move speed bonus helps counter one of the biggest drawbacks of the best Grappling race. And Free Action Strength boost is always nice.
Death Knights are good when working with others, maybe Sneak Attacking a Grappled enemy or helping to debuff something with Aura of Dread. There are simply too many big groups of enemies for them to compete with slightly better classes though. Aura of Dread is the only thing keeping them from being in a lower tier.
Storm Warrior works better when they lean into their casting a bit more, but at that point... you should just be a Druid.
And Champion may as well be a Cleric/Bishop instead.
B-: Gladiator, Samurai
Gladiator is fine in the early game with the combat Feats, but they're too limited as time goes on. If Blinding Strike can't work or isn't useful in a particular light, they're just gimped Fighters.
Samurai are kind of like Gladiators: a good early game class that tends to fall off later.
C: Paladin, Ranger
You'll notice these are only C and not D or F because none of the classes are really bad in this game. These classes are just a little too niche to compete as well with the rest, though they can still be good in the right group.
I actually like Paladins in KotC2, but objectively, I have to acknowledge that they're not the strongest class. They get moments to shine here and there with Smite, but later they're probably better off just casting Cleric scrolls so your more useful party members can do more important things.
Rangers are nice interrupters and of course great at ranged combat. But ranged weapons are limited in this game, and you lose all your equipment at a certain point. Add to that limited ammo and early game Cover penalties and they're just more limited than others.
So what are your thoughts? For reference, the guidebook ranks them: Wizard, Psionicist, Cleric, Druid, Bishop, Fighter, Gladiator, Warlock, Psychic Healer, Sorcerer, Bard, Psychic Warrior, Monk, Death Knight, Paladin, Ranger, Samurai, Storm Warrior, Champion, Mage Knight, Rogue, and Barbarian.