For pure novelty value and surprise, it's the Alliance quest that ends up with some Gnome coming in to land on a small island just off the coast of Azshara. I can't remember the name off-hand, but it's the end of a chain with no significant rewards and is a real blast.

Other than that, it's probably going to be the Marshall Windsor stage of the Onyxia attunement chain. After running Jailbreak <insert-large-number> times, it was initially relief and then absolute hilarity as I watched all the PvP twinks hanging around in Stormwind getting wiped out because they got too close.

I can't say that any of the Outland quests has captured my imagination at all to be honest, although poisoning the Scryer soldiers in Shattrath was worth a laugh