Well I was thinking about some of the awesome quests I've done (Loremaster and Seeker, so, heh) and was wondering what everyone's favorites were.
#5: DK Starting Area Chain
Heh, my DK is only level 62, but I really enjoyed the starting area for Death Knights.
#5: The Wrathgate Chain
http://www.youtube.com/watch?v=4QWR15OLcoI 'nuff said.
#4: The Worgen Quest chain in Grizzly Hills, particularly Escape From Silverbrook (Alliance)
http://www.wowwiki.com/Quest:Escape_from_Silverbrook... an epic escape sequence in which Worgen are chasing you through a dark forest and you armed with nothing but a torch? Epic fun.
#3: Mystery of the Infinite / Mystery of the Infinite Redux Chain
Getting a quest that starts players into the Nozdormu-is-leader-of-infinites-lore speculation, AND meeting your past and future self? Love that quest.
#2: the Matthias Lehner Quest Chain
Being able to play as Arthas and raise armies of undead, kill Illidan, AND entering Naz'anak The Forgotten Depths for the first time? Priceless!
#1: Banging the Gong!
Ah, the original realm-wide quest. I remember getting sooo much rep from cloth turn ins, and the process involved in actually creating the Scepter... so fun and amazing, if a bit time consuming.