Among the 16 mahajanapadas, Kasi was initially powerful. However, Kosala became dominant later.  

• A power struggle broke out between Magadha, Kosala, Vrijji and Avanti. Eventually Magadha emerged as the dominant mahajanapada and established the first Indian empire. 
• The first known ruler of Magadha was Bimbisara of the Haryanka dynasty. The Haryanka dynasty was succeeded by the Shishunaga dynasty. 
• The Shishunagas ruled for fifty years before the throne was usurped by Mahapadma Nanda.  
• Chandragupta Maurya Founded the Mauryan Empire after winning over Nanda Dynasty. 

Source: Tamil Nadu state board