Hellfire is my favorite zone. The landscape is pretty diverse, and it's where I did the bulk of my early world pvp.
My favorite zone is Nagrand. It's most beautiful zone in the game and many elite quests (including Son of Gruul). Also 1st Ring of Blood quests are in this zone good xp. Every time you down one of creature it would be one of your quests MOB :) almost no need to search quest mobs. Love it :)
In my opinion, it has to be Un'goro Crater. It's like the land that time forgot. A volcano in the centre, hot pools, tar pits, and dinosaurs. Visually, it's just stunning.
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nagrand has gotta be my favorite since its so easy to lvl through there.
Gilneas, because they're British. *monocle*
I really like the country/mountain culture feel and music of Grizzly Hills.
My favorite zones in the game are the "green" zones. Not all of them, mind you, but some of them. My top three are Nagrand, Howling Fjord, and Grizzly Hills. STV, Sholazar, and Twilight Highlands are distant seconds, while all other green zones are far, far behind. Nagrand, however, is something special. While it doesn't have the epic music of its Northrend counterparts, its music peaceful and relaxing. It has memorable quests like killing Durn, the original Ring of Blood, and the amazingly epic quest line involving Thrall, Garrosh, and the Greatmother. Don't forget Saurfang the Younger and Hemet Nesingwary! It has Halaa, which is still the best out of Halaa/WG/TB. It has an uninstanced FFA arena too!In short, Nagrand is steeped in lore, has amazing and memorable quests (talbuk poo, anyone?), and it a beautiful, relaxing zone where you really can't go wrong.
Darnassus. I love the quiet places, far from the crowded cities of Stormwind and Ironforge. I adore the almost-Greek architecture, the waters and bridges and the giant tree that now houses the Worgens. True, being on Darnassus isn't a road to anywhere but sometimes you just want a peaceful place to relax with any LD message going "Stormwind City is under attack!" again and again. Oh wait, the Horde have come to Darnassus ):
Easy: Storm Peaks. Even with the absolutely ATROCIOUS distances you had to fly, the lore was amazing, the quests were fantastic, and it was so well tied into the lore of Ulduar and the Halls that made it great.
My favorite zone would have to be Winterspring. Its unique because it was pretty much the first snow zone, and who wouldn't want an awesome mount for the alliance for killing bajillions of furblogs. The atmosphere is pretty great and even after the sundering it still has kept most of the essence that makes it unique and awesome.
My favorite zone really was Azshara before the Shattering. There wasn't much for me to do there as an Alliance, but I really loved the scenery and all the extra things to do there; my favorite of which was the Scepter of the Shifting Sands quest, having to talk to the Spirit of Azuregos. Over all, I just loved Azshara for everything that it didn't have. Classic Azshara will be missed!
Barrens back in vanilla was my favorite zone, barrens chat (that always ended up with Chuck Norris, alliance raids on crossroads and mobs being kited down just to smash the town.
Has to be the _OLD_ desolace. Don't really like the new one much.