"And by tough, you mean hard to kill, not hard to play. Right?"
Yes. My mage was abandoned by my little daughter, is only level 27, and I haven't gotten around to leveling it.
I have a level 85 Dk, but haven't gotten around to PvP-ing it yet.
I've done a lot of BG's on six other classes or so, and those are the two classes that seem hardest to kill.
(New note, priest is maybe the most OP healer for instances and BG's.)