Shadowmoon Valley. Upon release this zone was terrifying but sparse. It grew to include one of the most infamous characters in the Game (Illidan's home at Black Temple) and the quest chain for the Netherdrakes had to result in the ultimate mount in TBC, The Netherwing drake (save the armoured netherdrake). Between the sheer excitement of beating someone else to a Netherwing Egg and the thrill of the steeple chase races with the Orcs, this zone had rewards that far surpassed any other for the quest and grind junkies.
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My favorite zone is Icecrown specifically the Conflagration area where the undead soldiers are moving around. It feels good to put these brave souls to rest and release them from bondage. Plus killing any other servants of the Lich King is a bonus.
Magisters' Terrace. I liked the NPCs, every mob was a different class and you had to crowd control and focus silence them. Every pull was a little challenging which made it interesting. Also the music was amazing.
My favourite zone is Icecrown. It's pretty cool since its Arthas' domain and the mountains and ice make it look epic.
My favourite zone has always been The Isle of Quel'Danas. The only place I ever enjoyed logging on every day to do all my dailies. The Argent tournament was too spread out and Tol barad just doesn't have the same feel. But the Isle always brought me back with a fun little circuit around. As well as this I liked how this was one of those areas where the horde and alliance work together for once, as thats always an attraction to me seeing them put aside their differences.The whole are itself is also graphically pleasing too, a nice autumn colour scheme and lots of grassy areas with trees and beaches. And then there's the quests themselves. Bombing runs every day was always fun, and gave a nice fly over of the area too. None of the other quests were overly challenging or time consuming and there were two quests per questing spot which is always a nice plus.
SilithusIt genuinely feels like the edge of a continent with lots of strange, hard to explain phenomena going on in the whole zone, which really adds to the vibe of AQ20/40 because C'thun would use the edge of the world as a place to gather strength. The irreplaceable epic feelings of having the entire server work to open the gates years ago and having the huge multiraid bosses during the event also make Silithus a zone that will be impossible to ever replicate.
I have always loved Hellfire Peninsula. Mainly because back when The Burning Crusade was released it felt really huge to step through the Dark Portal for the first time, and the first thing you saw on the other side was tons of fighting demons. As a Warlock I must say it felt like coming home =)
Being a big fan of Lord of the Rings, my favorite is Icecrown. The detail that can be found in everything from the paving stones to the walls, to the ceilings is just dazzling. I also love the theme of Icecrown Citadel.
My favorite zone would have to be Molten Core. The reason I choose this is because what's better then traveling into the middle of the earth to fight giant fire bosses and getting items for a legendary weapon.
My favorite zone has got to be Vashj'ir. It's so colorful, and it's (i think) really innovative. Blizz did a really good job on it.
Deepholm. Because of Aeonaxx. :D
My favorite zone is Silithus. Despite being often overlooked, there is so much story and history to be found, there!