1. Organia. Great atmosphere. Fun to explore, except for the maziness. Not super deathful, unlike certain other exploration areas.

2. Ktengs. Good atmo, stuff to figure out. I'm positive I've only figured out a fraction of what's there, but then I sort of stopped trying.

3. Jade Vaults. Again, good atmosphere, lots of tricky things.

4. Thar Acacia maybe?

Hell was/is cool, but there's such a high barrier to entry (even disregarding the fact that it now requires staff signoff) that I never really got spend much time admiring it.

Trothon's like that too, to a lesser extent. Damn hard to find, and you're likely to die there unless you have buddies with you.