This little review will give you a good look at what you will be given when playing each class.
This will include the benefits and the downfalls.

Archer

The archer is a nice balance when it comes to Pdef and Mdef. It's a physical damage class working off of Patk.

Best Stats :
1) Power (gives 4Patk and 1 Pdef per 1 power)
2) Critical (Archers work mostly off of crits its how they gain rage fast)
3) Endurance/HP (They gain a passive 20% HP from skill tree)

What they excel at :

World boss : They are the single best Single Target class. This gives them the advantage at things like world boss.
Early game : They are very strong early game

Where they lack :

Multi-Target : They only have one "real" multi target which hardly does any damage for the cost. Their special for 100 rage will do 2-4 targets (random number) so not too reliable and costs a ton. Their multishot will always hit twice, though it might target the same one twice.

Late game : They start to fall behind really late game (50+), but can still compete well with the competition.

Mage

The mage is a really nice class specializing in AoE and support healing. It's the only class that has a skill'd heal that can target yourself and allies.

Best Stats :
1)Intellect (same thing as power, 4Matk and 1Mdef per 1)
2)Endurance (They lack a 20% HP buff so they fall a tad behind in the life department)
3)Pdef (Again they are great for Mdef but have hardly any Pdef which knights and Archers dish out)

What they excel at :

Crypts : They tear through crypts with their self heals and AoE's faster than any other class.
Early and Late game : They have a balance between Early and Late game giving it a smooth run through.
The only magic class : Giving the advantage of casting magic damage on targets with no resistances to it.
Healing : They can heal! They can support teammates and heal everyone up.
AoE : Multi-targeting spells.

Where they lack :

Pdef : They lack the physical defense that archers and knights dish out.
HP : They have the lowest HP out of all the classes (assuming you are geared to exact level of another person)
Cast time : Their spells have a slightly longer cast time than some skills of other classes.

Knight

Knights are a mix between Pdef and HP dishing out Physical Damage and putting up shields to block enemy attacks.

Best Stats :
1)Endurance : More HP (Your shield is 20% of your max HP, more HP more shield. Also gains a passive 20% HP boost.)
2)Pdef/Mdef : Defences against attack (take more hits and outlast the enemy, knights also have a passive 20% to Pdef)
3)Power : Patk (4Patk +1 Pdef per 1, not only do you get dmg you get armor too!)

What they excel at :

Late game : Knights are NOT early game characters. They really shine lvl 50+ (this is not for players who want early on power)
Tanking : You are the tank, you tank dungeons and arenas for your teammates while dishing out Medium amounts of damage.
Deleting rage : YES! The knight can delete another players rage! The intercept skill is a fantastic skill that some people overlook. It does a BASE amount of damage meaning it does that damage period (sometimes more) while also reducing the opponents rage by 30+.
Shielding : The knight can not only put up a shield for himself/herself for 20% of their life (yes 20% 1/5th of your life) but they can also give your whole team a damage reduction of 20%.
Fast rage : Knights get rage the fastest of all classes in the game to spam their skills.

Where they lack :

Early game : Again knights are not early game characters if you are not willing to dish out a ton of money to be the top character for a couple days. They are weaker before level 50 than the other classes but not by much.
AoE : They don't have any AoE other than whirlwind with its high cost of 50 rage. Reflect will hit anything that hits you (even multiple times) but other than that they are single target.
Rear enemies : Their abilities mainly target front line targets. They only ones who will reach the back are whirlwind and ultimate slasher (ultimate slasher targets the lowest % HP target, so if the back has less % HP it will target them)


NOTE : This is not a complete guide on everything, just helping pick a character.