Question: Who was the most famous emperor?

Lists of emperors

This is a list including all rulers who had carried the title of emperor through history.
Emperors of traditional empires
Ancient empires
Persian Empire (559 BC–330 BC)—see List of kings of Persia
Hellenistic Empire (331 BC–301 BC)—see Alexander the Great and Diadochi
Mauryan Empire (321 BC–185 BC)—see Mauryan dynasty
China Empire (221 BC–1911)—see 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
Middle Ages
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
Carolingian Empire (800–843, 884–888)
Holy Roman Empire (891–1806)—see List of Holy Roman Emperors
West Francia (875–877, 881–887)–see List of French monarchs
Bulgaria (Empire 913–1422)—see List of Bulgarian monarchs
Serbia (Empire 1345–1371)—see Emperor of Serbia
Muscovy (Empire 1547–1721)–see List of Russian rulers—continued in Imperial Russia from 1721 (see below)
Asian
Japan (monarchs known as tenno from 7th century CE)—see Emperor of Japan and List of Emperors of Japan
Korean Empire (1897-1910)-see Emperor of Korea
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
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)
Americas
Aztec Empire (1375–1521)—see Hueyi Tlatoani
Inca Empire (1438–1533)—see Sapa Inca
Pre-modern Empire
Spanish Empire was the first world-wide empire in history, (1492–1898) with its maximum expansion between 1520 and 1700 reaching 31 million square kilometers
Austrian Empire, 1804–1867 and the Austro-Hungarian Empire, 1867–1918 as Emperor of Austria (both under the Habsburgs)
British Empire de jure 1877–1947
India (under the British Raj with British Monarch as Emperor of India, 1876–1947)
Third Bulgarian Tsardom (internationally equivalent to a kingdom, 1908–1946)–see  list of Bulgarian monarchs
Empire of Brazil (Pedro I, 1822–1831 and Pedro II, 1831–1889)–see Brazilian imperial family
France: (under the Bonapartes, 1804–1814, 1815; 1852–1870)–see Emperor of the French
First French Empire ( Napoleon I, 1804–1814, 1815)
Second French Empire (Napoleon III, 1852–1870)
German Empire (under the Hohenzollern dynasty  as German Emperors, 1871–1918)
Russian Empire (under the Romanovs, 1721–1917)– see list of Russian rulers
Emperors of short-lived "empires"
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 )
Emperors Sunjong and Gojong of the Korean Empire (1897–1910)
The Hongxian Emperor of the Empire of China (1915–1916)
Emperor Puyi of the Empire of Manchuria (1934–1945)
Emperor Bảo Đại of the Empire of Vietnam (1945)
Emperor Bokassa I of the Central African Empire (1976–1979)
Self-proclaimed "emperors"
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)
Joshua Abraham Norton “Emperor of the United States of America” (1858-1889)
Fictional emperors

Galactic Empire (Isaac Asimov)
Emperor Sheev Palpatine of the First Galactic Empire (19BBY-4ABY)
Emperor Karl Franz, of the Empire of the Warhammer Fantasy franchise
See Category of fictional emperors and empresses