Between the Tolvir cities and ruins, the Ancient Egypt motif, and the Indiana Jones homage questline I'll have to go with Uldum as my narrow favorite.
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Nagrand is my favorite zone in the game. The zone is incredible to look at, and it has a calming feeling to it. Interestingly enough, the rest of the Burning Crusade zones are my least favorite.
My favorite zone is Hyjal.The fact that when you quest through it changes before eyes is awesome. Also, many of the areas look awesome and the variation is nice, with regards to how parts of it are completely forested and lush, while other areas are desolate with flaming monsters to fight. The change above ground to below ground is also cool because then you get to see the core of the earth and it is a nice preparation for the upcoming Firelands.
Vasj'ir. I love the swimming idea and it looks so relaxing.
My favorite zone has to be nagrand - It was big and colorful the quest hubs were amazing quite fun I thought and included Gruul's brother making it that much more fun slaying gruul himself (lolz I kill'd yer brother next you har har har next is your mother :D ) The PvP I enjoyed very much as I wanted to get the talbuk from the marks you earned only through pvp. Not to mention the awesome pvp atop the throne of elements as everyone would fight over the farming territory. It showed that open world pvp isn't dead as it became later in WoW.
Best place, one of the best moments is finding in Sholazar Basin a reference to the hit show Lost. With the Lost hatch making an appearance within the zone, even with the cursed numbers when mouseover'd :)
oh gosh. favorite zone....this is a tough decision. I will have to go with Vash'jir. Being a druid under the sea is amazing, running on the bottom of the sea in bear form is just downright AWESOME. I love the beauty of it. There's something about questing in the ocean, with big old baddies roaming around and so much to do-seahorsie quest? FANTASTIC!- I LOVE this zone. I would stay in it all the time if it had a capital city :D
I would have to agree with Nagrand as my overall favorite zone. Love the floating islands, especially the one with the dinosaur skeleton protruding from the side.I also loved Thousand Needles, pre-Cata, though most people probably wouldn't agree. Can't forget Hellfire Peninsula, if only because it's home to my favorite instance, Shattered Halls.
Vashj'ir is my favorite zone of any in the game, new or old. I love the lore, and am just a sucker for underwater enivronments, especially with ancient ruins in them. Can't wait for the next Old God and the rest of the stuff deep under the sea. Plus I'm a druid so I can go Ugly Form and zoom around it all. :P
The new Redridge is one of the most hysterical zones to quest through in Cata Azeroth and I love bursting through there on alts. It's even better when you continue the storyline in Burning Steppes. Keeshan is definitely the alliance's Saurfang!
My favorite zone would probably have to be Hellfire Peninsula. It just gives me that wonderful Outland-feeling I first got in Warcraft 3.
My favorite zone would have to be Teldrassil. Very mystical and the music always sticks with me