My favourite zone is, and always will be, Duskwood. It always has an eerie feel to it, and the quests work really well with the zone. Back in Classic, it was always interesting to try and dodge Stitches once he started pathing. I never level an Ally toon without running through all the quests there - and even at 85 I'll go back to run around, soaking up the atmosphere :)
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karazhan is one of my favorites. The area is so ominous and i love it for that.
Thousand Needles, post-shattering - Merberan is right. Leave it to the Goblins to make the best of their sunken race track.
My favorite zone is Hinterlands, It was an amazing quest hub in vanilla, specially with Jintha'alor, if you played a Horde melee class you'd get your first combat-useful trinket, and Nat pagle's fishing rod :D, which so many of us vendored in times of desperation.It was also one of the first times you could encounter high elves as Horde, except for that Captain in Ratchet and the escort guy out of Northwatch Hold(remember when they had skinny hourglass models?).And as Alliance, Aerie peak was a pretty amazing structure compared to the usual Alliance architecture, the Gryphon quests that connected Aerie Peak with Jintha was well made and scripted.there is also Seradane, when I went there on my 45ish Tauren warrior, I didnt expect to encounter a Mature Dragon for the first time.The Elwynn Forest music is also a beautiful piece of soundtrack, and it goes so well with the leveling pace.Whenever I'm leveling an alt Hinterlands gives me that warm, comfort feeling that you get when you are finally home after a long trip... I love that place.
My favorite zone is Orgrimmar because there are always people there and I never feel alone.
Scholazar Basin. The amount of time spent there farming herbs, ore, dailies, questing on alternative characters. It gave a great feeling of adventure and there were a lot of encounters you found there, PvP or just friendly passby's. The Basin is a rockin' back in Wrath on Dalvengyr. It was an amazing zone. :D
My favorite zone is the old Tanaris because we used to run around on our lvl 50 or so charactors ganking other players and having a blast. Also, I loved the Caverns of Time area in Tanaris because it always had a huge freakin dragon out front lookin all BA and what not.
Vash'jir, I love the underwater atmosphere.
Hillsbrad Foothills has always been my favorite zone since before the Wrath of the Lich King expansion, especially on Horde characters. Doing quests for the Forsaken Undead and performing their sadistic and brutal warfare on the relatively defenseless Hillsbrad population of human farmers and peasants, while simultaneously countering Hillsbrad's military defense patrols was always challenging at low levels. PvP servers would have even more craziness going on sometimes. At higher levels, attacking Southshore all by myself was occasionally a thrilling experience. At low levels, I used to love doing the quest to summon Helcular's Spirit (a relatively strong elite mob) and watching it attack noobs as they were leaving Southshore.I found post-Cataclysm Hillsbrad (again, as Horde) to be very exciting as well, but for different reasons--most notably, the part where you (the player) get to act as the quest giver and watch "adventurer" NPCs come by, talking and acting in similar ways to stereotypical players. I was incredibly amused by the "Dumass" character who accepts your first "quest." Not to spoil too much, but I will say this: It's laughable how many players I've encountered over the years who remind me of him!
My favorite zone has to be Ulduar, it was a simply awe inspiring zone and while the lore may have seemed thrown together to most it was still really well done.