I would have to say Indonesia. Simply because of the shear integration Indonesia has with Japan, it is a Democracy like Japan and the United States. Indonesia is the largest nation in ASEAN, over 256 million, the largest muslim nation in the world, and its success and orientation to Japan and United States will ensure stability. Second to Indonesia, is the Republic of the Philippines. I will have to say also that Both Indonesia and Philippines are the founding members of SEATO (now defunct) and also founding members of ASEAN.
Vietnam is a special nation to us, however relations are just beginning to develop.