I think best tankiness and durability actually goes to Necromancers. DKs are just better melee range brawlers who are built to be on the front line, survive it, and be able to deal damage. Their whole class kit is built around melee, they don't have many ranged options and rely on gap closers and speed to catch elusive foes, and most of their damage can be purged.
If you take away the Plaguebreak set I can guarantee you the DK class wouldn't be nearly as compelling to use. Once people can start purging all willy nilly again the DoT focused class is going to be at a massive disadvantage.... again.