Not really a single questline but the whole questing ensemble of Tirisfal Glades and Silverpine Forest.
On Alliance side the Duskwood quests are a close second.

I really like questing in each starting zone. Not even any zone in particular, all of them. Music and ambience in these zones are really well done. When almost every NPC around same race as you it really feels like home. You doing simple tasks at first, like bringing water to the Tauren village or collecting apples, up to the harder ones, eventually saving your capital from potencial danger. That's so cool. I've done these 1-12 zones like a >30 times each by now and never get bored!