Mahayana Buddhism is the most popular and alongside with Ancestor Worship, Taoism

Taoism

Taoism refers to a philosophical or religious tradition in which the basic concept is to establish harmony with the Tao , which is the mechanism of everything that exists...

 and Confucianism

Confucianism

Confucianism is a Chinese ethical and philosophical system developed from the teachings of the Chinese philosopher Confucius . Confucianism originated as an "ethical-sociopolitical teaching" during the Spring and Autumn Period, but later developed metaphysical and cosmological elements in the Han...

 as one which is the traditional faith of general Han Chinese

Han Chinese

Han Chinese are an ethnic group native to China and are the largest single ethnic group in the world.Han Chinese constitute about 92% of the population of the People's Republic of China , 98% of the population of the Republic of China , 78% of the population of Singapore, and about 20% of the...

 people (about 15% of total population of Brunei

Brunei

Brunei , officially the State of Brunei Darussalam or the Nation of Brunei, the Abode of Peace , is a sovereign state located on the north coast of the island of Borneo, in Southeast Asia...

).

From many estimates of different sources but the most common figure today is 14% of Brunei's population are Buddhists.

And Buddhism is also the second largest religion after state religion of Brunei: Islam.