I would run the Ramparts/Blood Furnace instances, for the quest chain.
The combo is pretty easy, and its fairly good rewards and experience.

If you are boosting...
Probably Ramparts 55-60, Underbog 60-63, Mana Tomb 63-65, Sethekk 65-67, Steamvault 67-68.
Then head to Northrend.

If you are running instances at the correct level (as in all the toons are the right level, no booster), I'd personally just quest. They have changed (reduced) the experience required for each level in Outland by 30%; my last team dinged 70th very early into Blades Edge Mountains. The toon with heirloom items the whole way actually reached 72nd prior to Blades Edge, which is pretty crazy. If you can get flying mounts, questing will probably blow away dungeons without a booster.