Who was the the supreme commander of allied forces in the pacific during ww2?

There was no single one, because there were three Theaters of War in "the Pacific" fighting the Japanese. These were the Central Pacific Theater of Operations, the Southwest Pacific Theater, and the China-Burma-India Theater. Each had its own commander, who took his orders from the Combined Chiefs of Staff (which was the American Chiefs of Staff and the British Chiefs of Staff - a "Chief of Staff" being the head of a branch of the service). The commander of the Central Pacific was Admiral Chester Nimitz of the US Navy, and the forces in his area were mostly the US Navy and its Marine Corps (the Marines created six divisions for WWII, all infantry, and all served in the Pacific), and some US Army troops (altogether there were about fifteen US Army divisions in the Pacific, divided between the Central and Southwest Theaters). Early campaigns in the Pacific (Guadalcanal, others in the Solomon Islands, Tarawa) usually involved one division at a time, and usually the division commander doubled as overall ground commander. (Guadalcanal was started by Marines, and finished by the US Army under General Alexander "Sandy" Patch, who went on to command the US 7th Army in France and Germany, making him one of the few officers to see action in both Europe and the Pacific). By the summer of 1944 entire corps of two or three divisions were involved, so there was a corps commander. Marines called their corps "Amphibious Corps" while the Army called theirs "corps", just as they did everywhere in the world. (In this usage a "corps" means two or more divisions, not to be confused with the Marine Corps, which is the naval landing force of the US Navy). Marine General Holland M. ("Howlin' Mad") Smith commanded the V Amphibious Corps at Guam and then the Fleet Marine Force at Iwo Jima.

The Southwest Pacific Theater was carved out for MacArthur, after he escaped (under orders) from the Philippines. The same consideration that led to his being ordered out of the Philippines also led to the creation of a Theater just for him. MacArthur was a former Chief of Staff of the US Army (1931-35) and it would be too much of a Propaganda coup to allow him to become a Japanese POW, and it would not do for him to have less than a theater command and be taking orders from officers he had outranked for decades. MacArthur had under his command the US 6th Army (General Walter Kreuger) and the US 8th Army (General Robert L. Eichelburger), some navy units and sometimes Marines, when he could borrow some. Both those field armies fought in the Philippines. The only other place in the Pacific large enough to deploy an entire field army was Okinawa, where the US 10th Army (which had both Army and Marine divisions) fought. The 10th Army was under Army General Simon B. Buckner (famous at that time as the son of the Confederate general who surrendered Fort Donelson to his old friend Ulysses Grant, which was the incident where Grant got his "Unconditional Surrender" nickname). Buckner became one of the two highest ranking American officers killed during the war (along with General Leslie McNair), and the only one killed by the enemy, killed by enemy artillery at Okinawa.

The China-Burma-India Theater was supposedly under the command of the Chinese "generalissimo", the warlord Chiang Kai Shek, of the Nationalists. The American General Joseph E. "Vinegar Joe" Stillwell was his deputy, but despised Chiang Kai Shek, whom Stillwell called "Peanut", or "Cash My Check". Stillwell continued in command of ground forces, in the complicated command structure of that Theater, but was replaced as Deputy by a very capable US Army General, Albert Wedemeyer, who was one of the US Army's foremost experts on the German Wehrmacht, having even studied at the German General Staff College - so of course, he was sent to China. A very capable British General in the Theater was William Slim, whom no one has ever heard of today.