Paladin can act as aoe-tank in classic and with classic tbc even better and got an "oh shit" button with his personal bubble and the bubble for a secondary character like your mage to do some nasty stuff every 3min.
Aside you got excellent single target heal and fancy buffs.

Priest can assist the mage best regarding single target damage with a shadow off or mainspec; unskilled shields/heals are still enough for a duo.

Druid... I dont know. He can tank, yes but struggles with large scale pulls. He has the weakest buffs for your purpose of the 3.
In healing he lacks the flexibility regarding a flash heal with his instant has a kind of long cooldown plus can't really help on big pulls except for spamming his 3sec heal.

IMO:
AOE farm: Paladin > Priest > Shaman > Druid
elite farm (1-3 mobs): Priest > Druid > Shaman > Paladin