It depends on what you define as a dreadnought - in naval historical circles, a dreadnought is any battleship with a uniform calibre main battery. There are some distinctions for later designs, with so-called superdreadnoughts having 15in or 16in guns, but they're still 'dreadnought-type' ships. The last such ships in operation were the American Iowa class, which served from 1943-4 to 1990-1. However, in colloquial use, a dreadnought is typically used to describe the battleships of the early-mid 20th century. Of these, the two longest serving ships were the Chilean Almirante Latorre, and the Turkish Yavuz. The first was constructed for Chile in the UK, and taken into the RN with the start of WWI as HMS Canada. Returned to Chile after the war, she served as a battleship until 1951, when she was turned into an oil storage ship following an accident. She served in this role until 1958, and she would be scrapped in 1959. Yavuz was originally the German battlecruiser Goeben, constructed in 1912. Goeben served as the German Mediterranean fleet, alongside the light cruiser Breslau. At the outbreak of war, both ships fled to the Ottoman Empire, and were taken into the Ottoman navy. She saw some combat against the Russians, and against British shipping. She served with the Turkish navy, in a greater or lesser state of repair, until 1950, when she was removed from active service. She would be struck from the Turkish navy register in 1954, but survived until 1971 when she was sold for scrapping.