Mine would have to be Elwynn Forest. I've done all the quests there way to many times and knew they all. With Cata some of them got changed and it just makes me like it that much more. Oh and Hogger. =)
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GM Island.I've never been there myself, but tales of glory and exploits from other aged players shine in the distance and provide that ever-longing desire to achieve greatness. Who knows if those stories are true... The rumors of the GM Chair and the multitudes of portals to take you anywhere you want to go! Perhaps someday, long from now when the world is old and broken, GM Island will be a new quest hub to revitalize old lands. You never know what will happen, but all we can do is gasp with awe at the expeditions of those (most likely now banned) who came back to tell their tale.
I would say Darkshore. Not only because it has a very nice mellow feel to it, but also (and mostly) because technically speaking the Dancing Troll Village (Shatterspear Vale) is in Darkshore. I always loved mountain climbing from Winterspring to visit those crazy trolls and their drum beats.
Hyjal, everything about it was awesome. The demigods, Malfurion, Ysera the background music, the quests (especially the one where you tossed baby bears out of trees), and of course the lore behind it all. It's almost too much to believe 4.2 is adding in a daily quest hub that will likely make the zone even better.Sholazar Basin comes in second for the almost-senseless yet awesome environment, in stark contrast to Storm Peaks (my third favorite). Of course, both zones have awesome and enjoyable quests.
My favorite zone is winterspring. The scenery is beautiful, the open pvp adds a little excitement and it is home to my largest time sink pre-catalsym... the winterspring nightsaber grind, which is so much easier now.
For me it has to be the new goblin start areas of kazan and the lost isles. The new types of quests there are truly enjoyable and inspired ideas. I've now made a goblin of every class just to experience how it plays for them all. The kaja-kola ideas are hilarious aswell "Tauren Paladins" yeh like that'll happen, oh wait!
My favorite zone is and I believe always will be Dun Morogh. My first character was a dwarf and I had a lot of fun going through Dun Morogh for the first time. It felt very much alive and akin to the races that lived there. I felt like everything belonged, as if everything depended on everything else. I've rolled alts just to play through it again, many times. I love snow.
My favorite zone is Mulgor! Shiny green grass, cows and a lovely lake.
Even tho I'm Alliance my favorite zone is defiantly Ghostlands. The area in front of Zul'Aman. I just love the feeling there, its very peaceful (even tho most seems t to be dead around you :p) And I made a lot of great memories there. Like playing hide and seek with some guild mates when we got bored and didn't know what to do. Sometimes I just wish there was an easier way to get there as Alliance, but then again that's what makes it even more special.
My absolute favorite zone is Sholazar Basin. The lore in the zone is fantastic. I like how Blizzard linked the zone with Un'goro crater. It was also where I found my first spirit beast Loque'nahak, which is the best spirit beast of them all. It is also home of the elusive Aotona, which is the best parrot skin out there. The way the quests just flow through the zone is remarkable. Starting in the zone, you start to enjoy the thrill of the hunt and by the end of the zone you have taken a side in a petty squabble between the Frenzyheart and the Oracles. You win whichever side of the argument to decide to join., although more so f you join the Oracles. Finally you push back the start of a scourge invasion with the help of Freya. Sholazar Basin is also the last home of the best hunter in the world, Hemit Nesingwary! The druids at D.E.H.T.A. will never be able to stop him on his hunting extravaganza!
Definitely Duskwood. Had that gloomy atmosphere and music that sent chills up your spine, along with hordes of creepy spiders and scary ghouls!