The obvious answer to this question would be accounting, but like so many obvious answers this is not necessarily the best one. Unlike the Certified Public Accountant (CPA) exam, which is mostly about public accounting and auditing, the Certified Management Accountant (CMA) exam is not all about accounting.
You have to have a decent to excellent understanding of micro and macroeconomics, statistics, general business, business ethics, and business analysis. You also have to have an understanding of more financially oriented topics such as budgeting, investment decision-making, and corporate finance. These are all in addition to the understanding required of financial accounting, cost accounting, and internal controls. Essentially, you have to be a jack-of-all trades and master of a few as well.
Going through a Master’s of Business Administration (MBA) program is undoubtedly the best preparation for the CMA exam. If getting an MBA prior to the exam is not possible for whatever reason, degrees in finance, economics, business, or accounting are all valuable preps for this certification. The accounting degree, unlike with the CPA exam, does not seem to be any better preparation for the CMA exam than these other degree programs.