I notice there's been a lot of concern about Dark Night currently, 59-soon-to-be-60 DRK main here. I'll go over the cards I'd most like to receive in order of priority.
1. Ewer. Yes, please. When MT, Dark Knight burns through their MP pool even faster than Black Mage. If you ever watch our MP bar during a fight it's a constant up and down, with only a few self-regenerating abilities available to us with Grit on. This job is all about MP management and being low at critical points can hamper our ability to tank effectively, such as having to round up a bunch of adds while not being able to use Unleashed. And unlike Black Mage, Ewer is less likely to alter our rotation in a bad way. Receiving a Ewer at almost anytime while MT would greatly effect my effectiveness over the next 30 seconds. When offtanking, Ewer is less important since we can use Blood Weapon to keep MP up more or less at full. However, if you or your co-healer is rapidly losing mana due to phases requiring heavy healing periods it is probably better to give this buff to yourself/them at that time; it could save the BRD or Machinist from using their refresh abilities and losing DPS.
2. Arrow. In short fights DRK really doesn't have any TP issues - none of our weaponskills cost more than 70 TP and our tools that are TP-intensive to other jobs instead use MP. Receiving Arrow would allow us to get more Syphon Strikes off to help MP regeneration. A good choice for us with minimal negative impact. If you're just using this for a general DPS increase though probably MNK would get a lot out of this buff as well.
3. Bole. A solid card for any tank, I feel this buff is most useful during the initial phase of big pulls or for tank buster moves, which should be obvious. Yet this wouldn't change how I approach a situation calling for damage reduction - I'm still going to pop cooldowns where appropriate - so I really think using this is more for your benefit since you're the one healing me. You could also spread this card and save it for predictable, damaging AoE with Expanded Royal Road.
4. Spire. Yeah I said before that Dark Knight doesn't have any TP issues. In short fights. I was doing Ramuh HM for a friend earlier today, and as MT I realized that I had completely gone dry about halfway through it. Ramuh HM is one of those fights that has absolutely no downtime for the MT since there are no tank swapping mechanics built into it. For fights like this, I will run out of TP eventually. Another poster in this thread said DRK and PLD were the only physical jobs with no form of TP-regen ability, and he was right. While this card can be a boon to other melee jobs like MNK, a DRK can still benefit. Our need for this is going to be on a per-fight basis so you'll have to use your own discretion as to the best choice for it.
5. Spear. Another tricky card for the reasons OP outlined. A lot of DRK's off-GCD abilities don't have super long cooldowns with the exception of Shadow Wall and Living Dead. Living Dead is an ability that is either planned at very specific times or an oh-shit button, both scenarios being hard to plan around a random buff system. There's more appropriate jobs to use this on. My guess is DRG, NIN, or hell even WAR would get more out of this than us.
6. Balance. Shouldn't have to be said, but this buff simply isn't for a tank class unless using expanded Royal Road. Give it to a DPS