It is generally admitted that the world’s largest oil spill occurred in Kuwait during the Gulf war in 1991 where around 10 million barrels of oil were spilled. The Kuwaiti oil fires were caused by Iraqi military forces setting fire to a reported 605 to 732 oil wells along with an unspecified number of oil filled low-lying areas, such as oil lakes and fire trenches. The fires were started in January and February 1991.
These numbers were significantly adjusted downward by later, more detailed studies. So the world’s largest oil spill is probably the Lake View gusher that happened in Kern County, California, in 1911.
However the world’s largest oil spills come perhaps from the endemic pollution that has spread over decades in the Siberian tundra and the Niger Delta. Unfortunately they are not measurable and no estimate could ever be made. They were often left unknown or silenced by poverty and politics.

List of the world’s largest oil spills of the oil history. Amounts are expressed in barrels (159 liters):

1- Russia (Chronic) No reliable data
2- Nigeria (Chronic) No reliable data
3- Lake View Gusher, California USA (1911) 9,000,000
4- Gulf war, Middle East (1991) 6,000,000
5- Deepwater Horizon, Gulf of Mexico(2010) 4,900,000
6- Ixtoc, Gulf of Mexico (1979) 3,300,000
7- Atlantic Empress, off West Indies (1979) 2,120,000
8- Fergana Valley, Uzbekistan (1992) 2,000,000
9- Kolva River, Russia (1994) 2,000,000
10- ABT Summer, off Angola (1991) 1,900,000
11- Nowruz, off Iran (1983) 1,900,000
12- Castillo de Bellver, off South Africa (1983) 1,860,000
13- Amoco Cadiz, off France (1978) 1,620,000
14- Mt Haven, off Italy (1991) 1,060,000
15- Odyssey, off Nova Scotia, Canada (1988) 1,020,000

(Image: oil wells in fire, Kuwait, 1991/NASA)