Here is the order i wish i did it in:

UK
Drak'Thalon
CoS
GunDrak
HoL (would recomend atleast 14-15k HP on every char and a strong aoe healer vs Loken, rest of the instance is pretty easy with 2 healers)
VH (Depends on the bosses ofc)
The Nexus
Old Kingdom
Azjol-Nerub
HoS (I barley managed the Brann event but its certainly doable, even in my gear!) Last boss could have been cleaner but i got the job done! :P
UP
The Oculus

OFC this vary ALOT on your gear and setup..

My setup was Protadin,2 elemental shamans,1 resto shaman and a survival hunter. Done me all and this is pretty much the heroic order i personally recomend althought everyone got theyr own opinion!