What is the best major in the business world in terms or post-grad salary and job availability?
There is no one answer. It depends on each individual’s aptitudes and interests.
In general accounting and IT are considered to be very employable options.
There isn’t a “best” major, as @happy1 mentioned it is an extremely subjective question even if you are trying to measure it by salary and job availability. If you are looking strictly at the broader business areas and strictly at entry level positions (marketing, operations, finance, etc.) then finance probably has the highest median salary followed by management/business strategy or economics, followed by accounting, marketing, and finally sales.
It’s more difficult to answer in terms of availability because there is a high supply of applicants and it is subject to each applicant’s credentials.