According to Al-Masudi, the greatest king of India was Balhara or Vallabhraja.

Vallabhraja, also known as Balhara, was a ruler of the Gurjara-Pratihara dynasty, which was one of the prominent dynasties in medieval India. He ruled from the late 8th century to the early 9th century CE.

Vallabhraja is praised by Al-Masudi in his book "Muruj adh-Dhahab wa-Ma'adin al-Jawhar" (The Meadows of Gold and Mines of Gems) as a great king who possessed immense power and ruled over a vast empire. Al-Masudi describes him as a wise and just ruler who brought prosperity and stability to his kingdom.

Vallabhraja's empire extended from the Indus River in the west to the Ganges River in the east. He successfully defended his kingdom against foreign invasions and expanded his territory through military campaigns. His reign is considered a golden age for the Gurjara-Pratihara dynasty, with the capital city of Kannauj becoming a center of art, culture, and learning.

During Vallabhraja's rule, the Gurjara-Pratihara dynasty reached its zenith, with the empire becoming one of the most powerful in northern India. The dynasty was known for its military prowess and administrative efficiency. Vallabhraja's reign saw the construction of numerous temples and monuments, showcasing the rich architectural and artistic traditions of the time.

Vallabhraja's patronage of learning and scholarship also contributed to his reputation as a great king. He supported scholars and intellectuals, leading to a flourishing of literature and knowledge in his kingdom. This period saw the rise of renowned scholars like Vatsyayana and Shankaracharya.

In conclusion, according to Al-Masudi, Vallabhraja or Balhara was considered the greatest king of India due to his wise and just rule, military achievements, expansion of his empire, and patronage of art, culture, and learning. His reign marked a golden age for the Gurjara-Pratihara dynasty and left a lasting impact on the history and culture of India.