I've gotten told off for mentioning this before but my favorite test of tanking skills is HM TC adds. Assign your highest DPS to the adds and simply blow them up from the start. No matter the class its a challenge to control two adds and interrupt at the same time and it forces you to react as opposed to following a set rotation. My opinion at least.
Absolutely. IMO the HM TC adds are the hardest threat challenge in the game right now, unless there's something in NiM TFB/S&V I don't know about.