Strangely enough, I actually had to think about this one. I used to enjoy Theramore Isle pre-expansion, and Duskwood as well (both Alliance side). Since Cataclsym, Stonetalon Mountains is great for the Horde and funny at times. But once I stopped and really considered all the zones in the game, Storm Peaks became the obvious answer.
We’ll overlook how long it took for me to get an Arctic Fox Kit to complete Northrend Safari, because that has nothing to do with the questing in the zone…merely my own bitterness. No, I think back to when I first did Storm Peaks. How enjoyable the quest chains were. Brunnhildar Village, Sons of Hodir (even though I know many didn’t like them), and of course the build up to the Ulduar related dungeons. To this day I feel like a twit for not noticing Thorim and Loken. But the Loken manipulation line is so good and includes my favourite quest in the game, which is The Drakkensryd.
Add in that it’s a winter zone, with a great score, and enjoyable dailies, and you have what I would easily consider the “sure fire” zone for my enjoyment.