When I lived in Taiwan from 1996-98, it seemed as if Japanese popular culture was making the biggest impact on East Asia, but now South Korea looks to be stepping in as the driving force of the entertainment industries of not only Northeast Asian countries, but also in Thailand, Vietnam, Malaysia, Mongolia, and Uzbekistan:

South Korea, historically more worried about fending off cultural
domination by China and Japan than spreading its own culture abroad, is
emerging as the pop culture leader of Asia.
From well-packaged television dramas to slick movies, from pop music to
online games, South Korean companies and stars are increasingly
defining what the disparate people in East Asia watch, listen to and
play.