Who was the greatest king in India, and what did he do for India so that he is called famous?

There are a couple of ways to answer this.  One
possibility would be the Emperor Ashoka, who ruled most of India from 269BC to 232BC.
His empire was the largest India knew until recent times, although he expanded his rule
in a series of violent conquests. After he stopped warring he converted to Buddhism and
adopted a philosophy of peace and nonviolence. Under Ashoka’s rule, Buddhist teachings
were spread far and wide across India and beyond.

Another
possibility would be the Mughal Emperor Akbar, who ruled from 1555 to 1605. He engaged
in conquest during his reign as well, and also consolidated and strengthened Mughal rule
in India. Though Mughal rule was Muslim, Akbar maintained policies of religious
tolerance over Hindus and others in his empire and he encouraged religious and
philosophical debates at court. His own religious views bordered on the heretical, as
later in life he promoted a sort of “cult of personality” centered on himself and
encouraged nobility and others to treat him as a semi-divine
being.

This is admittedly a subjective question. I don’t
know whether we want to consider conquest a mark of greatness. I chose these examples
because they did later embrace ideas and values that promoted peace (in Ashoka’s case)
and tolerance (in Akbar’s case), and whose legacies included increased stability over
the parts of India they ruled.