Burma—recently ended military junta, State Counsellor and Nobel Peace Prize winner Aung San Suu Kyi is overseeing the genocide of the Rohingya Muslim ethnic minority
Cambodia—de facto one-party state, which was formerly a Marxist-Leninist party
Laos—communist
Thailand—military junta ended last year, questionable elections
Vietnam—communist
TL;DR Every country in Southeast Asia is authoritarian.
List of authoritarian states in Southeast Asia:
TL;DR Every country in Southeast Asia is authoritarian.