The 1 Ring

The 1 Ring can only be found by fishing.

Source
This item can be found by fishing in any low level fishing area, such as the canals in Stormwind and the ponds of Orgrimmar. However, it has an extremely low drop rate and even an avid fisherman is unlikely to ever see one.

Mr. Pinchy's Gift can also contain this item.

Trivia

 * Direct reference and humorous variations on The One Ring, the most powerful item in Middle-Earth, which drives the plot of . Being found by fishing references the discovery of the One Ring by the hobbits Déagol and Sméagol while on a fishing trip.
 * The 2 Ring jokingly referred to in the item's tooltip would later be implemented in patch 2.4.0.