Its Welcome to the Machine. We all know that it is. But I know that some people haven't done it - I myself dislike the Forsaken and so I resisted doing the quests in Hillsbrad for a long time. But eventually I succumbed and what so many told me was true - this quest is the best quest in World of Warcraft.
But since I understand that not everyone is going to agree, and because I'm always interested in what other players think, let me put the question to you - what do you think is the best quest in World of Warcraft, if for some reason you don't think it's Welcome to the Machine (which it totally is)?
I'm also fond of Fiona and the paladin pals from Cataclysm and the overall Klaxxi experience in Mists of Pandaria, if we need runners' up. Vanilla Darrowshire and Order Must Be Restored would be my favorite classic quests. So now it's up to you - share your favorite quests, the best quest in the world (funny thing is that quest isn't actually anything like this quest, this is just a tribute).