I recommend the following:

Quests 1-10
SFK 10-20
Scarlet Monastery 20-35
Zul'Farrak 35-45
Scholomance 45-55
Hellfire Ramparts 55-58 (Blood Furnace is also a wonderful place)

Hellfire Questing at 58-60 to gear out everyone

Either quest or continue to BC powerlevel from there.