For Medica as I stated before, because the MP cost is about 3 cures worth, you want to make sure it's doing that much healing. Also keep in mind it heals each person a bit less than a cure.
Here's what I put in my WHM guide for debuffing: In brayflox some of the issue is more people grouping up too much and being needlessly poisoned, if they're not dying I would just prioritize on self and tank.
Silence (Self) - this should be a no-brainer, but have Echo Drops on hand in case of silence.
Disease - Disease doesn't come up much until the lvl 50 dungeons but it sucks, your heals that usually do 1k+ will be doing only around 100 HP so cleanse this before curing.
Poison - It hits REALLY hard at lower levels, on the tank however, you will want to cure and then hit esuna as soon as you can that way you are at less risk of the dot and the mobs from taking him/her out.
Paralyze - It’s annoying, and on top of interrupting actions it can also stun you as you try to run out of fiery death.
Pacification(On the tank) - Pacification prevents weapon skills from being used, which for the tank means no cooldowns and no enmity gains. (Note Warriors using Beserk will have Pacification on them frequently so this can be hard to judge)
Pacification/Silence (Damage Dealers) - Keep in mind Silence isn’t as important to cleanse from Pugilist/Monk or Lancer/Dragoon, the same goes for Pacification for Thaumaturge/Black Mage and Arcanist/Summoner.
There are a few other Damage over Time debuffs that you can cleanse if there’s not a lot of other damage going on. (Depending on the fight they may be same priority as poison).
Blind - While this hampers the dps, you may want to cleanse this on the Tank to help with Enmity, otherwise low priority.
Otherwise, prioritize curing over cleansing.
When there are multiple healers in the group you should discuss who will focus on Esuna, especially on high debuff with high damage fights.
--
Once you have Medica II becareful with it for threat but it's a better option than Medica, Medica will then be best used during BIG hits where you'l use Medica II and then Medica right after.