Comfey is unique in doubles in its niche of providing speed control with priority healing support. While its HP isn't all that good and its offences are also quite poor, priority healing is very useful, being able to heal a partner before an enemy can strike it to finish it off regardless of speed tiers can be a game changer, especially if Grassy Terrain is active.
Comfey @ Sitrus Berry / Babiri Berry / Mental Herb / Grassy Seed
Calm nature
Ability: Triage
252 HP, rest in Def/SpDef
- Draining Kiss
- Giga Drain
- Floral Healing
- Tailwind / Trick Room / Grassy Terrain
Draining Kiss and Giga Drain deal roughly the same amount of damage after STAB, but Draining Kiss heals Comfey more. Giga Drain provides slightly different coverage and becomes stronger under Grassy Terrain, and both moves get priority thanks to Triage, allowing Comfey to go first regardless of speed tiers or Trick Room. Floral Healing heals Comfey's partner and gets an extra boost in Grassy Terrain to heal more. Tailwind and Trick Room both provide different kinds of speed control, it depends on your team which is most beneficial, although having both on the same set is generally ill-advised. Some teams benefit more from Tailwind, others more from Trick Room. Note that Tailwind has to come from USUM tutor. Comfey itself doesn't really care one way or another, all of its other moves getting +3 priority and going first regardless. Grassy Terrain is another option for the final move if the speed control isn't needed as much, allowing Comfey to set it up itself and remove Psychic Terrain which would block Comfey's Draining Kiss and Giga Drain (as you usually don't want to Dynamax Comfey). Sitrus Berry gives Comfey a bit more staying time on the field, but can be replaced with Babiri Berry depending on the opponents that are popular, or even Mental Herb to negate Taunt once so you can still set up Trick Room or heal. Grassy Seed is also an option to increase Comfey's bulk if you have a reliable Grassy Terrain setter in your team. Max HP is the primary EV investment for Comfey as its base HP is not that great, investing a lot of EVs in it gives it a significant boost to bulk. And since its primary purpose is supporting rather than attacking bulk is generally preferred over special attack.
As partners, Rillaboom sets up Grassy Terrain and offers Fighting coverage to deal with Steel types. Heatran can also be very good, benefitting from Grassy Terrain weakening Earthquake and Heatran can come in easily on Steel, Poison, and Fire attacks that are aimed at the Grass types that like Grassy Terrain being up, while being threatening to steel types itself. Heatran benefits from the frequent heals that Comfey can provide, giving it a big boost to its bulky nature, and also likes either Trick Room (for slow Quiet builds) or Tailwind being up. Alolan Exeggutor is a bit of an oddity, but it likes Trick Room and its Grass moves can set up Grassy Terrain when Dynamaxed. Exeggutor also has Flamethrower to threaten Steel types, and can set up sun with that Flamethrower when Dynamaxed, which can combine well with its Harvest ability (if running berries on it).
Flower Shield may seem like an interesting option, but only giving a +1 in a defensive stat is quite poor boosting, and in the worst case it could boost an enemy as well. It's also limited to Grass-type partners. Aromatherapy, Protect, Dazzling Gleam, Substitute, all largely better options than Flower Shield, even aside the ones I listed in the set.
Pollen Puff is largely inferior to Floral Healing. Although it can also be used offensively, losing the Triage priority on it is too much of a loss and it also doesn't benefit from Grassy Terrain.