I would try them all before you decide. I play all three and IMO
PLD- the most easiest and safest to tank with and high survivability, horrible DPS though, but that will change in Stormblood thankfully. If you value protecting your allies more than anything else then this is your job
DRK- Meeehh ok....very MP dependent but also fairly easy to get argo. I don't like this job as much as the other three but it does have good survivability
WAR- Very fun and awarding, it's pretty much a DPS that can tank and does great damage. Takes a little bit of time to learn how to argo cause it's a cone aoe attack you will use ("Overpower") so you have to try to force the mobs to bunch up and very TP dependent. They are taking away some of WAR's healing abilities...but all in all it's still going to be the main offensive tank in SB
