Hardest role in group content

Derra

Healer

Healer if you´re actually a good one providing all needed buffs for your grp.

Asardes

Healer

Yep, healer and tank are responsible with the buffs & debuffs of the group. If the tank just taunts and blocks and the healer just spams BoL or healing springs they clearly fail at their role.
Assuming DK tank and Templar healer:
Tank buffs: ingenous weapons, aggressive warhorn, various set items (ex. Ebon Armory)
Tank debuffs: pierce armor/noxious breath, crusher enchantment, heroic slash, choking talons
Tank CC/interrupt: cinder storm/razor caltrops, unrelenting grip (or invasion if target can't be pulled)
Healer buffs: combat prayer, any dawns wrath ability, aggressive warhorn
Healer debuffs: elemental drain, various set items (ex. Infallible Mage)
Healer CC/interrupt: shards, crushing shock (some mobs need to be interrupted from range)