By zone, not really in order top to bottom.
Kael - Avatar of War
ToV - Lord Vyemm or Aaryonar probably
ST - Hraashna if I remember right
Icewell - Dain Frostreaver IV
Plane of Growth - Tunare
DN - Zlandicar
Skyshrine - Yelinak
Outdoor/Other - Sontalak, CT (post revamp)
That's more difficulty than loot. CT and Vulak drop the best items pretty much.
Top of my head, some may be a little off it's been a while.
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