Scepter of Shifting Sands chain
Did this at 80, and while I'm sure it was a complete pain in the ass at level I thought it was cool how you had to go to pretty much every corner of Azeroth. I really miss the South Seas islands - the desolate atmosphere of islands out in fatigue waters was very cool. Also, am I a bad person for waiting until the Lunar Festival to complete the green flight part of the chain?
Stiches quest chain
I really miss seeing this guy stomp around in Duskwood - really made the world feel alive
Scholomance key chain
I really liked the concept of forging a key to enter a locked dungeon - it really made it seem like the dungeon was dangerous.