What was general douglas macarthur's first victory with the allied troops in korea?

The Battle of Inchon is the correct answer.

The Battle of Inchon was an organized amphibious invasion that allowed the United Nations army, lead by McCarthur, to gain control of a key portion of North Korea near the 38th parallel.In this battle, they defeated the North Korean army.

 The invasion/successful capture of this area allowed the United Nations army to recapture the capital of Seoul from the North Korean Army just a few short weeks later.