Bolstering the Effectiveness, Success, and Transparency of Ambassadors Act or the BEST Ambassadors Act This bill expands disclosure requirements for individuals nominated to serve as an ambassador or in a related position. If the President's nominee to serve as a chief of mission is not a career member of the Foreign Service, the President must provide to Congress a justification explaining whether a career member was available and why the nominee is uniquely qualified to serve in the position. The President must certify that any campaign contributions made by an individual (or the individual's immediate family members) played no role in the decision to nominate the individual to serve as a chief of mission. The nominee must also provide to Congress an explanation concerning how the nominee meets the criteria for performing the position's duties. A nominee to serve as a chief of mission, ambassador at large, or minister must report to Congress political contributions made by the individual or immediate family members in the 10 calendar years preceding the nomination (currently, the report only requires reporting of contributions from the preceding 4 calendar years). The Department of State must make this information available on its public website.