What countries were most affected by Japanese advancement in World War 2?

If you take "affected" to mean "suffered the greatest damage", then, that would be the Chinese, by quite a large margin. From a Chinese perspective, WW2 was merely the latter part of the Second Sino-Chinese War, which had begun in 1937. A huge portion of eastern China was occupied by the Japanese, who burned, pillaged, and raped their way through most of it. Total Chinese casualties for that war are 20+ million, second only to the USSR in total casualties for WW2.

In addition to China, the following countries suffered considerably at the hand of their Japanese occupiers, who often carried out campaigns of rape, torture, and murder against their conquered populances:

• The Philippines - they suffered extensively, both due to a Japanese contempt for the Filipino race, and by the fact that many Filipinos joined their American overlords in active resistance to the Japanese occupation
• Korea - where the historical contempt of the Japanese for Koreans reached an all-new high (or, low, depending on how you consider slavery, murder, and forced rape)
• Burma, Thailand, and French Indochina (modern Vietnam and Laos) - tends of thousands were used as disposable labor in a variety of Japanese projects, most notably the "Death Railway" in which over 90,000 Burmese Thai, and Laotians died.
• Singapore - as a former crown colony of the British Empire, the treatment of Singaporean citizens was nothing short of criminal.

While the US, France, Britain, and the Netherlands lost colonies or possessions, these were not generally critical to those countries (except perhaps in terms of access to certain raw materials, rubber and oil being most important).