Pick whatever flavor you like honestly…although two of the four tanks are locked behind certain requirements. Gunbreaker requires you to have reached level 60 on any job and Dark Knight requires you to reach the Heavensward-expansion. A quick breakdown:
Warrior (starting class marauder): The short version? Imagine being a Warcraft-warrior but you’re permanently a tank with a big, heavy axe. You have the self-healing of fury, you have the “Press this button to do heavy damage” of arms and you’re tank like protection.
Paladin (starting class gladiator): Well it’s a paladin what do you expect? You have your holy magic, you have your sword and shield, and lots of glowing particle effects including giant wings which boosts your parties defense if they stand behind you. Also you flat-out get Divine Shield. It’s like a combination of Retribution-Protection-Paladin with a bit of Holy (you can heal party members for example).
Gunbreaker: You’re not a tank. You’re a DPS who pretends to be tank. You’re rotation is pretty much closer to that of a DPS-job, and involves using special-enchanted-cartridges to pump out damage.
Dark Knight: And now my personal favorite and honestly my main job and as such I will write more. The edgelord-job. You get a big-two-handed-sword, lots of red, black and purple particle effects with edgy names to get along with it (Souleater, Delirium). This class starts out somewhat weak compared to the others until level 70 when you unlock “The Blackest Night” and suddenly get access to the best mitigation spell in the entire game.
But, even if you want to play any other tank you should still unlock Dark Knight because the job has arguably one of the best storylines in the entire game. It’s so well written that you could add the class story into the main plot of the game…and it would make perfect sense. I honestly could write several paragraphs about why it’s so good, but I really, really don’t want to spoil anything of it because especially the level 35-50 part is absolutely amazing. My eyes actually got wet during the final parts is all that I can say. Yes, it’s that amazing.