Warrior, if you wanna be the healer and Tank at the same time in dungeons, and it's the easiest to play because it's main source of damage is 1 Button spam, using a 2nd/3rd boosts, with a couple O-GCDs. Warrior has been king of dungeons since the introduction of Nascent Flash, and still is.

Dark Knight, requires skill if you wanna survive dungeons, I wouldn't recommend for new players. Playing the job, you need to master double weaving without slowing your Global Cooldowns, weaving as in off-global cooldowns. Off Global cooldowns operate on timed cooldowns, global cooldowns are the 2s+ delay attacks and combos you build upon. It's a job that's basically a 50 Cal LMG, so much power, a pain to reload.

Gunbreaker is a combo tank, meaning you will be single/double weaving a lot, less skill required to play, the whole plan for the tank is using everything in it's No Mercy window before the buff falls off.

Paladin is easy to play, but at the same time high skill ceiling for optimising the job, you must follow a rotation order, in order to play the job well. Paladin is really strong in dungeons after reaching 90 after the blades boost, it's Blade of Valor DOT tears trash mobs apart.