I'm not sure that there is a definitive "most efficient" job for farming, and the answer is going to perpetually be in flux as the game (and meta-game) evolve. Certainly every job has pros and cons that need to be considered, and which you use is likely to be a very personal choice.
Of course, this discussion is really only relevant for high level mobs. Attacking a level 1 mob at level 60, with any job, is going to be a one-hit kill, and you're simply not going to be hit enough for defense to be a concern. For high level targets, your available gear, and skill at each job will probably end up being the dominant factor here, but I'll quickly go over my take on each job and it's farming capacity, assuming your skill and gear are equal.
Disclaimer: I primarily play PLD so my opinions are probably biased toward PLD and tanks in general. I've tried to keep this as unbiased as possible, but, as I have more experience with tanking than anything else, the details are still skewed to some extent. I also want to make it clear that these are strictly my opinions on soloing. My views about party content differ considerably.
Tanks are obviously the most sturdy, and the easiest to keep alive, but their damage may be lacking compared to other jobs. Tanks are also going to be getting a relatively major adjustment with the changes to VIT and STR landing in patch 3.2, but other than changing the gear requirements, I don't think this will have much impact on farming.
- PLD: By far, the most defensive job in the game. PLD can put out reasonable damage, but it requires very good gear to do so, meaning it will probably lag behind in terms of speed. PLD gains an incredibly strong healing spell called Clemency at 58, but is only really able to cast it twice before running out of MP. PLD is also due to get an offensive buff in patch 3.2, so that may change things up quite a bit. PLD is a good choice if you're worried about being killed.
- WAR: Has an absolutely ludicrous HP pool, but is otherwise weak defensively, WAR makes up for this by having a large arsenal of self-healing abilities. WAR is also the most offensively capable of the tanks, so it's a good choice if PLD isn't fast enough for you.
- DRK: DRK occupies a wierd middle ground between PLD and WAR. It's less offensively capable, and has fewer self-heals than WAR. At the same time, it's less defensive than PLD. DRK is commonly called a "magic damage tank" but this is probably not ever going to be a factor for farming. If you already have DRK leveled to 60, it's a fine choice, but I would probably choose PLD or WAR instead if you're looking at using a tank to farm.
Melee DPS have much higher damage output compared to tanks. However, all of the melee DPS have some level of positional dependency, meaning standing in front of the mob is going to adversely affect their damage output to some degree. They are offensively quite week, but can offset some of this with low-potency self-heals. You may, occasionally need to wait for HP to regenerate between kills.
- DRG: DRG does solid damage, and has good survivability compared to the other DPS. It has access to three self-heals, and it's less positionally dependent than MNK. This would be my choice of the Melee DPS.
- MNK: The most positionally dependent of any job in the game, but despite this, they can put out incredibly high damage. Weaker defensively than DRG, and has access to two self-heals.
- NIN: The most mechanically complex of the melee DPS. Has high damage output, and about equal to MNK defensively. Only has access to one self-heal.
Ranged DPS benefit from being able to easily kite (as long as Wanderer's Minuet and Gauss Barrel are not used). They are generally weaker offensively, but have strong AoE and DoT damage, making group kills easier.
- BRD: Not much to say that I didn't say before. Much weaker defensively than the melee DPS. I would probably pick BRD over MCH.
- MCH: Unfortunately, MCH is the weakest DPS in the game offensively. But, this will hopefully change in 3.2. They do have the benefit of having turrets, and if you happen to be farming in an area where the mobs are tightly clustered, turrets might be very beneficial.
- BLM: Very strong AoE damage. Reasonable single-target damage. Defensively weak, but has access to physick for some weak healing ability. As you have noticed is hindered by cast times making it difficult to dodge and deal damage at the same time.
- SMN: Arguably the best DPS in the game as of 3.1. SMN has by far the best DoT and AoE damage of any job in the game. Spike damage is more difficult. Has an incredibly diverse set of options for soloing, thanks to their pets; Titan-Egi can tank, Garuda-Egi and Ifrit-Egi can deal extra DPS. They also have access to physick, for healing. This is a very good choice for farming.
Probably the worst choice for farming due to their lackluster damage output coupled with poor defense.
- WHM: Holy makes WHM an interesting choice for farming. It's an AoE damage spell that stuns the mob. As long you don't get crazy and pull too many mobs it's probably a viable option. Other than that, and fluid aura (single target knock-back) WHM doesn't have very much to offer.
- SCH: SCH has many of the benefits that SMN has thanks to being derived from ACN. It's significantly weaker offensively than SMN is, but having your fairy out to heal frees you up to DPS. I would say SCH is probably a better choice for farming than WHM. However if you have SCH leveled you, by default, have SMN leveled as well, because they share a base class. The only reason to pick SCH over SMN for the purpose of farming is if you don't have any SMN gear.
- AST: Um... I can't really come up with any reason to use AST for farming. It's perfectly adequate in a party situation, but pretty much any other job will outperform AST solo; they can solo, but you're really better off picking something else.