The southern coast of Arathi Highlands is mountainous and all but impassable. Most players leveling through the zone in vanilla never bothered to explore beyond the steep ridges. Yet if you were curious, you might have discovered a tucked-away area known as Faldir's Cove. To find it, you either had to swim along the coast or discover a small cave tucked away in the hills southeast of Stromgarde. The area wasn't labeled on the map, and no NPC sent you there. Explorers were rewarded with perhaps the least-known quest chain in vanilla.
Other secret quests such as Message in a Bottle were "hidden" in plain sight in high-traffic areas. You were bound to notice The Matron Protectorate if you ran Blackrock Spire enough -- or someone would helpfully point it out to you while you were grouped. The only one that might be more obscure was Sully Balloo's Letter, but that wasn't really a line of quests, and you didn't do anything but talk to some NPCs. Therefore, I give the title of most hidden quest line to Faldir's Cove.
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