What was the one that started with the abandoned boat in Un'goro? Yea, that one!
Man if I could only remember, there were many I enjoyed. Gonna love doing them over again fresh from the start so I can remember.
I have always liked some of the quests in Duskwood, The legend of Stalvan, Morbent Fel and Mor'Ladin, i like the atmosphere and the spookyness of the area.
Everything this guy said and I mean all of it!
But also the Tier 0.5 quest chain was really fun and long!
Jail Break!
If you want to do Onyxia you gotta do this chain quest.
The escort itself is a pain the arse but the other parts of it are fun.
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!