Depends on your allegiance (Alliance or Horde) profession and spec. For miners I would do Southshore / Arathi. For leatherworkers and armorsmiths, Stranglethorn Vale.
Southshore (Alliance): take the path up from Wetlands, do the quests at the entry point, and get the Refuge Pointe and Southshore FPs before you pick up the quest on Thandol Span (let me know if you find the location of the quest bottle in the water!).
From there, do the Southshore quests, until they run out/get too hard (typically the Arathi Highlands bit) and then do some in Refuge Pointe.
For Stranglethorn (Alliance), pick up the FP in the Rebel Camp, work south.
For both, if you want the achievement, do the Nessingwary quests in SV.
At level 40, you then have the choice of Dustwallow Marsh, Shimmering Flats, Hinterlands, Tanaris, Searing Gorge and Swamp of Sorrows.
Level 50 Ferelas, Felwood, Burning Steppes, Western Plaguelands and Blasted Lands.