I would argue that the most important pieces of federal labor legislation are the Wagner Act and the Civil Rights Act of 1964. The first of these made it much easier for unions to organize while the second set up the Equal Employment Opportunity Commission, which has been given a mandate to enforce laws against discrimination on such grounds as race, sex, age, and disability.
These pieces of legislation have been important because they have to some extent bound the hands of businesses over the years. We have seen the effects of union contracts on the auto industry and we can argue that anti-discrimination laws have been the cause of many lawsuits such as the gender discrimination suit against Wal-Mart.
From a business perspective, then, these are two of the most important pieces of federal labor legislation.