Step 1: Analyze the sentence.
The sentence is: “Sardar Patel was one of the greatest man of India.”
Step 2: Grammar rule.
The phrase “one of the …” is always followed by a plural noun. Example: “one of the best players,” “one of the greatest leaders.”
Step 3: Identify the error.
Here, “man” is singular. It should be “men” to make the sentence correct:
“Sardar Patel was one of the greatest men of India.”
Step 4: Conclusion.
The incorrect word is “man.”
Final Answer:
\[
\boxed{\text{man}}
\]