I don't really have an absolute favorite, but there are a couple that really stand out.

The Rift - I just went all Skyrim fan-girl once I hit that area. And the snow! Whenever I PvP (AD) whenever we reach the northern zones we're all like... "Omg so pretty snow!!"

Grahtwood - You don't realize how large Elden Root is till you get there. There's so much to explore in the environment also, between the trees, along the rivers, over the cliffs... *kerplunk*

I do love the architecture and European-esque style of the Daggerfall zones, though I haven't seen it all in detail.

After doing the first two zones in the Ebonheart pact I was actually missing the woods of the Dominion... homesick much? xD