Who was the most powerful ruler of Mauryan empire?

Ashoka

Many of these historians found the message on Asokan inscriptions very different from that of most other rulers, suggesting that Asoka was more powerful and industrious, as also more humble than later rulers who adopted grandiose titles.

The Mauryan empire was founded by Chandragupta Maurya. Its capital was Pataliputra. Asoka was the grandson of Chandragupta Maurya and is regarded as one of the greatest rulers of ancient India. The assassination of the last Mauryan ruler Brihdaratha by his commander-in-chief- Pushyamitra led to the end of Mauryan Empire  c. 185 BCE.