In terms of sheer wealth, Mansa Musa I of Mali is widely considered to be the richest man who ever lived. In today's money, his estimated fortune would be worth over $400 billion.
Mansa Musa was a 14th century ruler of the Malian Empire, which at its height stretched from present-day Senegal in the west to Niger in the east. Mansa Musa's wealth derived primarily from the empire's vast gold reserves, which were said to be so plentiful that the streets of Cairo ran with gold after he made a pilgrimage there in 1324.
In addition to his enormous personal fortune, Mansa Musa was also incredibly generous and is known for giving away much of his wealth to those in need. He built numerous mosques and madrasas (Islamic schools) throughout his empire and also provided financing for many public works projects.
While there have been other individuals throughout history who have been extraordinarily wealthy, Mansa Musa's fortune is unrivaled and he remains the richest man who has ever lived.