In 1346 Stefan Uroš IV Dušan proclaimed himself a Tsar (meaning "emperor", ultimately related to Latin Caesar). The empire was short-lived; Serbia suffered a major defeat at the hands of the Ottomans in the Battle of Kosovo in 1389 and became a vassal state.
Over the next several decades Serbia was ruled by princes who held the title of Despotes (a Greek word, from which we get the modern term despot). Serbia was finally conquered by the Ottomans in 1459 and would not be fully independent again for 400 years. During this time Serbia was often a battleground in the wars between the Ottomans and the Kingdom of Hungary and later the Habsburg Empire.
In the early 19th century, with the Ottoman Empire on the decline, the Serbs revolted. Serbia became a kingdom again in 1882.
The final three kings in these lists were rulers of Yugoslavia, of which Serbia was only a portion. The last king, Petar II, was deposed by the Communists.