Any place that has minimal quest chains or content funneling. Any place that just lets me explore the area on my own with no further "You need to go this way now" quests.
Unfortunately the mid-level zones are very hit and miss in this regards. There are area's that are very rigged in set up, forcing you to follow a very tightly predetermined path. Then there are area's that basically say "go figure it out yourself", those are the most true to previous TES games, and those area's I enjoy the most.
But in terms of visuals and environment, so far I'd say Shadowfen is very well made and has me looking forward to the potential addition of Black March (plenty of references are made by NPC's suggesting it could well be added at some point).
I do like how geographically the world seems to make a lot of sense. Many other MMO's have their environments change randomly from zone to zone, from dry desert to snowy mountains, which breaks immersion. ESO does this right: The transition of environments from Stonefalls, to Deshaan, to Shadowfen feels very natural.
Are the Aldmeri regions all trees and other green stuff? I've actually thought about maining a Daggerfall or Ebonheart character instead for the deserts or the ashlands and snow regions. Too much forest bores me .
There is this outcrop in Auridon, basically just a pillar of rock with some grass and a couple of trees on top to which you can get only if you sprint-leap from a cliff high above and land in one of the trees just right so you don't get hurt. Else the fall kills you.