I think Desert of the Flames and Rise of Kurnark are my personal favorites. Although Kingdom of Sky has beautiful places too. Mad'Dul City is very beautiful . Freeport Revamped is amazing.
I remember being just stunned the first time I was in there. I need to go back at level 100 so I can just LOOK.
By the time Sentinel's Fate came out, I thought there was no way a new zone could really amaze me. Then I saw Vasty Deep: Conservatory. It's just amazing. Screenshots don't do it justice; it really needs to be seen "in person" to be appreciated.
I've always been partial to Kylong Plains. Something about stark and harsh looking areas then go to Terens and be in snow or travel a little further and be in what it would feel like to be in a giant forest lush and warm (probably humid too ugh). I love it there, some zones I get all huffy about having to travel through them to get to where I'm going, never get that feeling in Kylong.
Mistmoore Castle is also pretty jamming. And if you are an old school Quake player, you can not deny getting a smile on your face the first time you went into abandoned labs.
Conservatory is up there. I also like nek forest. Estate of Unrest also. Funny thing is ... EQ1 had a worse graphic engine but I remember more amazing looking zones from that game.
LFay for me in terms of beauty. But for a zone that makes you feel like you're really there, Ferrott. Between the music, the bugs sounds, and the feeling of heat and dampness, it's amazing.