In my opinion, the Berserker class is the best for the story in Zenonia 5. Berserkers have high physical damage output and can tank quite a bit. Their skills are very effective in combat situations that often occur in the story, like when facing hordes of enemies. They can quickly clear a path and progress through the story levels more efficiently compared to some other classes.
Well, I think the Paladin class is excellent for the story in Zenonia 5. Paladins are a balanced class with good defense and decent attack power. They have healing abilities that can come in handy during the story's battles. When you're following the story and getting into tough fights, the Paladin can not only hold their own but also keep themselves alive long enough to complete the objectives. Their holy-based skills also fit well with the overall theme of the story in many cases.
The Mechanic class is a great choice for the story in Zenonia 5. Mechanics can use various ranged attacks which are useful in different story scenarios. They have the ability to deal damage from a distance, which gives them an advantage in some of the more difficult battles where getting up close might be dangerous. Also, their skills can be upgraded to have a wide area of effect, which helps a lot when facing groups of enemies in the story.