Well, I guess it's (unsurprisingly) unanimous for Nimitz at this point. I'll add my vote.
MacArthur was a vainglorious ass. He was totally unprepared (as others have said), and should have been crucified for bailing out at Bataan while his under-Generals and troops were taken as POWs. He was tactically unremarkable (except for Inchon), and in fact made some grave post-Inchon blunders in Korea. He even advocated using nuclear weapons to create a miles-wide barrier of radioactive wasteland between North Korea and China to prevent the Chinese from intervening on behalf of North Korea. After he was relieved of command by Truman, the US forces rallied, managed to extricate themselves from a very bad position, and were able to retake the territory south of the 38th parallel.
After WWII, he had a famous Japanese General, the "Tiger of Malaysia" (I forgot his real name) executed at war tribunal, falsely accusing him of being responsible for the rape of Manila, when in fact it was members of the Japanese Royal Family that were responsible. Although MacArthur knew the truth, he needed to execute the Tiger because (A) he embarrassed MacArthur by being a significantly superior tactician--beating him at Bataan and then later holding out for months against an overwhelming force under MacArthur, and (B) by scapegoating the Tiger, he could avoid blaming Hirohito, and thus have greater success putting over his constitutional reforms with cooperation from the Emperor.
His best skill was at political infighting within the military and putting forth a shiny image of himself. He signed letters to his wife "--Gen. Douglas A. MacArthur".
In short, he was a lousy general and a despicable human being.
