That's the funnest way to level, doing dungeons at the level they were intended for. Still really easy since heirlooms and stat squish stuff but fun. A few snags along the way, outlands dungeons get too repetitive so you might go quest for a bit here and there.
As for your tank choices, nobody really complains about boxing any of the tanks this expansion. I'm running Pali this time, others loving the mobility of the warrior tank, and I forget why people love the bear and the brewmaster.
But if you want a generalized rank of tanks. I'd say Palis then warriors are the easiest to box, dks and brewmasters the toughest.
Edit:
Here is a general idea about the different tanks, referring to solo players but helpful to boxers.
Here is a link to the pros and cons of each tank. Again its about solo players but useful to boxers.Warriors: Mobility & Offensive Utility
Paladins: Healing & Defensive Utility
Druids: Evasion & Flexibility
Death Knights: Self-healing & Personal Defenses
Monks: Self-healing & Evasion
http://us.battle.net/wow/en/forum/topic/7924333197#4
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