Fischer's bio on wikipedia is well worth reading:
It contains this statement, "Now aged 23, Fischer would win every match or tournament he completed for the rest of his life."
On the face of it that sounds incredible, but given that Fischer retired after winning the world crown in 1972, it transpires that his absolute dominance lasted only from 1966 to September 1972. One also has to take into account that he withdrew from some contests.
Even so, I think he was the greatest.