After the end of Forty Years' War in 1424, Ava gave up its dream of rebuilding the Pagan Empire. Instead, for the next six decades, it would struggle to hold on to its vassal states. Rebellions arose every time a new king came to power. The new king would have to reestablish his power all over again by gaining the fealty of all the vassal states, usually by show of force. Of these, the remote vassal state of Toungoo , tucked away in the southeastern corner across the Pegu Yoma range, proved most troublesome to successive kings at Ava. Toungoo lords rebelled against Ava in 1426-1440, 1451-1459 and 1468-1470, usually with Pegu's implicit or explicit support. Ava also faced rebellions at Mohnyin  and Prome . The beginning of the end of Ava came in 1480. The new king Minkhaung II was greeted with a multitude of rebellions but it proved to be different this time. The most serious rebellion was by his younger brother, lord of Yamethin. With a serious rebellion so close to Ava, vassal states broke away one by one. The Yamethin rebellion went on until 1500. In 1482, Minkhaung's uncle at Prome successfully revolted, the Prome Kingdom sprang into existence. Ava's vassal Shan states of Mohnyin, Mogaung, Momeik and Kale also broke away in the 1490s. Surprisingly, Ava's steadfast ally during this period was the usually restive vassal of Toungoo which remained loyal to Ava until Minkhaung's death in 1501.
Question: Who is proved most troublesome to successive kings at Ava?
Answer: lord of Yamethin; Toungoo