Fadeleaf
Fadelead requires 150 skill points in herbalism to gather and is most likely to be found under bushes and near other plants, so zones with plenty of vegetation is the place to be to farm Fadeleaf. Zones that yields to most Fadeleaf would be Dustwallow Marsh followed by Stranglethorn and Arathi Highlands.
See below for maps of where to farm Fadeleaf in WoW.