The answer is Death Knight, Demon Hunter and then Mage/Warlock in that order.
Death knights are immune to magical attacks, are undead so they can shrug off physical attacks on top of them wearing saronite armor. They have rune blades to cast powerful magic and there’s a reason the ebon Blade have been the bulwark of Azerothians going into the shadowlands.
The only thing that makes demon hunters weaker is while they’re also strong versus casters they don’t have the benefits death knights have from being undead.