This is a list including all rulers who had carried the title of emperor through history.
Persian Empire (559 BC–330 BC)—see List of kings of Persia
Empire of Alexander the Great (334 BC–281 BC)
Mauryan Empire (321 BC–185 BC)—see Mauryan dynasty
China (Empire 221 BC–1911)—see Emperor of China and Table of Chinese monarchs
Funan (68–550)—see List of Kings of Cambodia
Roman Empire (27 BC–476)—see List of Roman Emperors
Ethiopia (Empire ca. 3rd century–1975)—see Emperor of Ethiopia, Rulers and heads of state of Ethiopia, and Kings of Axum
Western and Byzantine traditions
Byzantine Empire (395–1453)—see List of Byzantine Emperors—and its rival & split-offs,
Latin Empire of Constantinople (1204–1261, with the title of (Latin) "Emperor of Constantinople" continuing to 1383)
Empire of Nicaea (1204–1261)—"Byzantine Empire" in exile during the rule of the "Latin Empire" over Constantinople.
Empire of Trebizond (1204–1461)—Another split of the "Byzantine Empire"
Empire of Thessalonica (1224–1242)—Offshoot of the Despotate of Epirus, reduced to vassal status of other powers after 1230
Ottoman Empire (1299–1922)—see Ottoman dynasty
Holy Roman Empire (800 and 962–1806)—see List of Holy Roman Emperors
Bulgaria (Empire 913–1422)—see List of Bulgarian monarchs
Serbia (Empire 1345–1371)—see Emperor of Serbia
Muscovy (Empire 1480–1721)—continued in Imperial Russia from 1721 (see below)
Chola Empire (300–1279)—see List of Tamil monarchs
Chenla (550–802)—see List of Kings of Cambodia
The first ruler of Vietnam to take the title of Emperor (Hoang De) was the founder of the Dinh Dynasty, Dinh Bo Linh, in the year AD 966—see List of Vietnamese dynasties
Japan (monarchs known as tenno from 7th century CE)—see Emperor of Japan and List of Emperors of Japan
Khmer Empire (802–1431)—see List of Kings of Cambodia
Ottoman Empire (1299–1922)—see Ottoman dynasty
Mongol Empire (1206–1634)—see List of Mongol Khans
Persian Empire (Iran) (1501–1979)—see List of kings of Persia
Mughal Empire (1526–1857)—see List of Mughal emperors
Durrani Empire (1747–1823)
Talpur dynasty (1783–1843)
Aztec Empire (1375–1521)—see Hueyi Tlatoani
Inca Empire (1438–1533)—see Sapa Inca
Austrian Empire, 1804–1867 and the Austro-Hungarian Empire, 1867–1918 (both under the Habsburgs)
British Empire de jure 1877–1947
India (under the British Raj with British Monarch as Emperor of India, 1876–1947)
Empire of Brazil (Pedro I, 1822–1831 and Pedro II, 1831–1889)
France: (under the Bonapartes, 1804–1814, 1815; 1852–1870)
First French Empire (Napoleon I, 1804–1814, 1815)
Second French Empire (Napoleon III, 1852–1870)
German Empire (under the Hohenzollern, 1871–1918)
Russian Empire (under the Romanovs, 1721–1917)
Emperor Jacques I of the Empire of Haiti (1804–1806)
Emperor Augustine of the first Mexican Empire (1822–1823)
Emperor Faustin I of the Empire of Haiti (1849–1859)
Emperor Maximilian of the second Mexican Empire (1864–1867)
Emperor Sylvain I of the Empire of Haiti (1868–1870, not confirmed [1])
Emperors Sunjong and Gojong of the Korean Empire (1897–1910)
The Hongxian Emperor of the Empire of China (1915–1916)
Emperor Bokassa I of the Central African Empire (1976–1979)
See also Self-proclaimed monarchy and micronation
Simeon Uroš, "Emperor of Serbs and Greeks" (held by Uroš V), in reality Despot of Epirus and Thessaly (1359–1369)
John Uroš, "Emperor of Serbs and Greeks", in reality Despot of Thessaly (1369–1373)
Jovan Nenad, "Emperor of Serbs" (1526–1527)
Paul of Smederevo, "Serb Emperor" (1560s)
Galactic Empire (Isaac Asimov)
See Category of fictional emperors and empresses
Lists of emperors Wikipedia (Text) CC BY-SA