S-Tier
The Priest sits firmly in S-Tier due to being the only character that is Mandatory to use in your team no matter the setup you choose to play.
She offers unparalleled support through boosting the rest of your teams damage via Blessing of Might as well as strong healing via Heal and Sanctuary.
This combined with the ability to build as an extreme tank, you will for sure want her in your line up.
A-Tier
Here in the A-Tier there are a few options for you to play around with depending on your Character Availability and Preference.
The Sorcerer is a solid Starter Character offering Great AoE Damage and later on excellent Single Target Damage via Flame Hydra.
The Ranger starts of weak but scales her damage extremely quickly due to having access to a phenomenal amount of Attack Speed, allowing her to shred enemies both Single Target and Groups alike.
With the ability to also stack a lot of Dodge Chance later into the Endgame, the Ranger also offers a decent level of Defence for a Damage Dealer which comes in handy later down the line.
If you have access to the Hunter then she can be used as a replacement to the Ranger.
Playing more into Elemental Damage and harder hitting Ranged Attacks as opposed to attacking Giga-Fast, she will annihilate any foes that are thrown your way so long as you have a sufficient Tank in place to distract Enemy Damage whilst she deals with them.
B-Tier
The Slayer sits in B-Tier and is a perfectly playable option, however, he doesn't offer the same level of Defence that the Priest or Knight does, lacks the Support of other Character Classes and is required to be in Melee Range to deal damage so can be susceptible to Dying until geared with High End Equipment.
It is recommended to take one of the A-Tier or above Classes to begin with and then transition into the Berserker later on when you have some Equipment to make proper use of him, if you choose to have him in your team.
C-Tier
The Knight currently sits down in C-Tier due to lacking Damage and offering Mediocre Defensive Measures that won't help save you without excellent Equipment.
He is essentially a worse Priest right now in terms of being able to act as your Tank, doesn't offer the sort of support she does and you absolutely want Priest in your team, so you are left sacrificing an Offensive Character to have both the Knight and Priest in your team.
For this reason he will likely be forgotten right now in favour of better options, although if you really want to play him, he will be more than viable with the right Equipment.