What is the class with best AoE?

The best thing for what you want is a mage, probably cleric or I/L since they can farm by clicking one button and literally nothing else to level from 50-80. It's not easy to screw it up, just know that LUK is completely useless and a noob trap so don't add it.

Warriors are not good for afk farming, they need to move around a lot because they are melee. Would not recommend if you want an afk playstyle

 

Any mage works since they all have map clearing skills in 4th job. The only warrior with something similar to that is dark knight with dragon roar in 3rd job but it uses 30%-59% of your hp every time you use it depending on its level. Warriors also don't have any mobility so farming becomes even more sub-optimal.

 

You want one of the archmages. F/P does more damage than I/L but I/L is more afk if you can't OHKO the enemy thanks to blizzard freeze (F/P can also freeze with summon but it can be more finicky)