Best Tank for Druid as Healer

[Lune]

Monk

Reasoning:

Knights aren't even a part of the discussion, because they bring no DPS, mitigate only marginally better than a monk, and their utility doesn't make up for the lack of dps. Rangers aren't particularly well-suited to a druid partnership. Even though they bring superior agro management and dps, their lack of indoor single pulling and their relative physical weakness compared to monks makes them an inferior choice.

Warriors mitigate better than monks, but do a bit less dps. They can't single pull in dungeons, and lack mend. Whatever small advantage warriors have in sturdiness/agro management, will be more than offset by their inability to single pull and their lower dps.

Monks bring an overpowered compromise between DPS and mitigation. They are both sturdy and deadly. They single pull more efficiently than a shadow knight under current mechanics in most content, plus mend is nice for emergencies or baseline efficiency. Outdoors, the monk is able to use Intimidate to (somewhat unreliably) fear non-animals, which the druid can snare and attack with a charmed pet.