Havoc dh. Their defenses are bad. Their damage is weak.
Going to take this as hardest to master:
Healers: Rdruid and Disc
DPS: Feral and Rogue. Would say Mage too but feel as though the skill ceiling for mage has been heavily lowered in SL and BFA.
Just my opinion.
What? Ret pally is hardest by far dudeee