Who was the king of the vanara (monkey) tribe?

The king of the vanara tribe was Sugriva. In the Ramayana, Sugriva ruled the vanara kingdom of Kishkindha and became one of Rama’s most important allies.

Sugriva was the younger brother of Vali, who originally ruled Kishkindha. A tragic misunderstanding involving the demon Mayavi led Vali to believe that Sugriva had betrayed him. Vali then drove Sugriva into exile and took Sugriva’s wife, Ruma. Sugriva found refuge on Rishyamukha mountain, where Vali could not safely go.

Rama met Sugriva during his search for the abducted Sita. They formed an alliance: Rama would help Sugriva regain his kingdom, and Sugriva would provide the vanara forces needed to search for Sita and fight Ravana. After Rama killed Vali, Sugriva resumed the throne of Kishkindha.

Jambavan was a wise bear-like leader, Angada was Vali’s son and crown prince, and Vali was Sugriva’s elder brother and predecessor. Those relationships make them common alternatives, but not the answer to this question.