Vvardenfell for the nostalgia
Northern Elsweyr for the ambiance and well written story
And the zone story for Malabar Tor. I enjoyed the struggle to find the new Silvernar & Greenlady (also helped raise my Bosmer lore since I play as a Bosmer)

I'll say Murkmire. The main quest line was fabulous. I loved the way the game developed the great characters in that questline. Most significantly, my elf and Jasik-Orn started off hating each other but over time and very naturally - beautifully even - became very close friends. Beyond that the indigenous Argonians welcomed my Bosmer pretty much throughout the zone and she was happy to help them - a far cry from her reception from the Dark Elves and High Elves of Morrowwind and Summerset respectively.