I did instance:

70-71 Utgarde Keep
71-72 Nexus
72-72,5 Azjol-Nerub (It was decent exp, but couldn't live without loot!)
72,5-75 Ahn'kahet
75-78 Gundrak (did it around 30 times)
78-80 All level 80 instance.

I did Drak'tharon Keep and Violet Hold aswell, but only 1 or 2 times, but they sucked exp wise so went back to the others.