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.

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

The correct answer to your question is of some considerable debate. It should be remembered that India has long been divided North and South between Islamic and Hindu cultures. Both have long been fragmented.

My own opinion is that the greatest king of India, although not long lived 606-648 C.E), was King Harsha, also known as Harshavardhana. He came to the throne after the collapse of the Gupta dynasty at age sixteen, and subdued most of Northern India with an army of 50,000 men and 5,000 elephants. Although a Buddhist, he was tolerant of other religions, and patronized scholars liberally. His greatest asset was perhaps his personality; however local traditions and customs in Northern India were too entrenched. He was assassinated in 648 and left no heir, after which the area broke apart into warring states again.

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

The greatest King of India was Emperor Asoka.   His empire included the Indian sub-continent as well as what we now call Pakistan and Afghanistan. Asoka is known as Asoka the Great because he was one of the best rulers that ever governed India. During his tenure all of India was united. The government ran smoothly. He excelled in everything that he did. His name, Ashoka means "without any sorrow" in Sanskrit.  He was born in 265 B.C. and he was crowned the king of India after the death of his father.  He proved that he was an excellent ruler by controlling his government and keeping the territories united.

At one point in his rule he decided to conquer Kalinga.  After the battle Ashoka viewed the dead bodies, blood and terror that war caused.  He vowed never to fight again and converted to Buddhism.  He is credited to be the first Emperor to make a serious attempt at developing Buddhist policies and spread his belief of Buddhism all over the world.

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

What about the Mughal Emperor Akbar?  He reigned from 1556 to 1604.  You could argue that he was the greatest because he brought the Mughal Empire to its greatest size.  It was while he was emperor that the Mughal Empire came to directly rule almost all of Northern and Central India.