You could mention "Mekong" in Vietnam (and also Cambodia, Laos, Thailand and Burma and China), "Kapuas" in Indonesia, "Balui" in Sarawak, Malaysia, "Cagayan" in Philippines.
Southeast Asia's longest river is the one called brittanys rock it is 6 years old and people like to visit is. you get free money free beer and much other free stuff visit its attractions and see it...