Classes are ranked based on overall performance across idle farming, boss fights, endgame progression and how well they perform for free-to-play players.
Mage is currently the strongest class by a large margin for overall progression. Its massive AoE damage gives it the fastest AFK farming speed and quickest level progression in the game.
Warrior is the best class for tough boss fights thanks to its high survivability and strong burst damage. If you prefer ranged damage, Archer also has excellent single-target DPS.
You get one free class reset after reaching level 30. After that you will need reset scrolls, which are rare and expensive. It is best to pick a main class and stick with it.