My favorite zone is Nagrand, as I'm sure many other have said. The quest chains were fun and enjoyable and offered some interesting pvp back in the day. It was also just a well design and beautiful environment. Deepholm is also becoming a favorite of mine because of the wonderful environmental design.
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I like Grizzly Hills because it's just so peaceful looking and it reminds me of a trip I took to California once. Also the music isn't too bad there either. (Grizzly Hills, not California)
The new Azshara. The quests are awesome!
I'd have to personally say that my favorite zone is Nagrand. I believe that it is the most beautiful and well designed area in not only all of outland, but Azeroth itself. It's just so earthen, so to speak, elementals and just natural raw beauty. The floating islands add a nice "Outland-ish" touch that was a fresh and awesome touch back when BC was first released.
Id have to say my favorite is Hillsbrad Foothills post Cata. The quest chains out there are hilarious, and it has that fun plants vs zombies mini game.
Mine is definitely Elwynn Forest. Ill never forget how epic the zone felt since it was the first zone i played in back in vanilla. Everything felt new and shiny. Every weapon and piece of armor i got felt like i was getting the best item in the game.
Grizzly Hills because the music was amazing and it was one of the most entertaining zones in the entire Wrath expansion.
Uldum without a doubt! I felt like a grown-up Short Round helping Indy around. I dropped a little nostalgic tear :)
Shadowmoon Valley is the cream of the crop. Danger lurked around every crag and gully. If you weren't careful - no matter how geared - you could get smashed in a hurry. The amazing quest line to get attuned for Black Temple was fantastic - you really felt for the broken Draenei. The netherwing drake questline was also epic - you needed mad flying skills to complete it. Best of all, the Black Temple sat menacingly to the east.
It's impossible to have a favorite zone in WoW I think. Each zone has a particular feeling or memory associated with it, either from back when you were Night elf just starting out and was amazed by the zone being crafted onto a gigantic tree, maybe it was when you first went to Outland and was amazed by the wide variety of zones, from the dunes of Tanaris to the tropical forests of Stranglethorn, exploring the Japanese-era Nagrand for hours, the jagged and elemental ridden Deepholm, Auberdine before it had been swallowed by the ocean and all of its denizens, and all the other-countless areas and memories we have of Azeroth is what makes WoW.