The worst genocides of the 20th Century
Mao Ze-Dong (China, 1958-61 and 1966-69, Tibet 1949-50) 49-78,000,000
Jozef Stalin (USSR, 1932-39) 23,000,000 (the purges plus Ukraine's famine)
Adolf Hitler (Germany, 1939-1945) 12,000,000 (concentration camps and civilians WWII)
Hideki Tojo (Japan, 1941-44) 5,000,000 (civilians in WWII)
Ismail Enver (Turkey, 1915-20) 1,200,000 Armenians (1915) + 350,000 Greek Pontians and 480,000 Anatolian Greeks (1916-22) + 500,000 Assyrians (1915-20)
Pol Pot (Cambodia, 1975-79) 1,700,000
Kim Il Sung (North Korea, 1948-94) 1.6 million (purges and concentration camps)
Menghistu (Ethiopia, 1975-78) 1,500,000
Yakubu Gowon (Biafra, 1967-1970) 1,000,000
Leonid Brezhnev (Afghanistan, 1979-1982) 900,000
Jean Kambanda (Rwanda, 1994) 800,000
Suharto (East Timor, West Papua, Communists, 1966-98) 800,000
Saddam Hussein (Iran 1980-1990 and Kurdistan 1987-88) 600,000
Tito (Yugoslavia, 1945-1987) 570,000
Fumimaro Konoe (Japan, 1937-39) 500,000? (Chinese civilians)
Jonas Savimbi (Angola, 1975-2002) 400,000
Mullah Omar - Taliban (Afghanistan, 1986-2001) 400,000
Idi Amin (Uganda, 1969-1979) 300,000
Yahya Khan (Pakistan, 1970-71) 300,000 (Bangladesh)
Benito Mussolini (Ethiopia, 1936; Yugoslavia, WWII) 300,000
Mobutu Sese Seko (Zaire, 1965-97) ?
Charles Taylor (Liberia, 1989-1996) 220,000
Foday Sankoh (Sierra Leone, 1991-2000) 200,000
Slobodan Milosevic (Yugoslavia, 1992-96) 180,000
Michel Micombero (Burundi, 1972) 150,000
Hassan Turabi (Sudan, 1989-1999) 100,000
Jean-Bedel Bokassa (Centrafrica, 1966-79) ?
Richard Nixon (Vietnam, 1969-1974) 70,000 (vietnamese civilians)
Efrain Rios Montt (Guatemala, 1982-83) 70,000
Papa Doc Duvalier (Haiti, 1957-71) 60,000
Hissene Habre (Chad, 1982-1990) 40,000
Chiang Kai-shek (Taiwan, 1947) 30,000 (popular uprising)
Vladimir Ilich Lenin (USSR, 1917-20) 30,000 (dissidents executed)
Francisco Franco (Spain) 30,000 (dissidents executed after the civil war)
Fidel Castro (Cuba, 1959-1999) 30,000
Lyndon Johnson (Vietnam, 1963-1968) 30,000
Hafez Al-Assad (Syria, 1980-2000) 25,000
Khomeini (Iran, 1979-89) 20,000
Robert Mugabe (Zimbabwe, 1982-87, Ndebele minority) 20,000
Rafael Videla (Argentina, 1976-83) 13,000
Guy Mollet (France, 1956-1957) 10,000 (war in Algeria)
Paul Koroma (Sierra Leone, 1997) 6,000
Osama Bin Laden (worldwide, 1993-2001) 3,500
Augusto Pinochet (Chile, 1973) 3,000
Al Zarqawi (Iraq, 2004-06) 2,000
(Note: the crimes committed by right-wing dictators have always been easier to track down than the crimes against humanity committed by communist leaders, so the figures for communist leaders like Stalin and Mao increase almost yearly as new secret documents become available. To this day, the Chinese government has not yet disclosed how many people were executed by Mao's red guards during the Cultural Revolution and how many people were killed in Tibet during the Chinese invasion of 1950. We also don't know how many dissidents have been killed by order of Kim Il Sung in North Korea, although presumably many thousands). Main sources:
* Charny (1988) Genocide: A Critical Bibliographic Review
* Stephane Courtois: Black Book on Communism (1995)
This does not include the misery of those that were not killed. Don't worry useful idiots will keep looking for that one right person that can make communism/socialism work.