What is the safest method of payment in an international business?

Letter of Credit (LC) is the best instrument to secure a payment as in the sales transaction international or reputed banks become gurantor for the payments towards goods and services provided.

Confirmed irrevocable letter of credit.

To get a "Letter of Credit"  Opened in your favor through the Importers' Bank

Letter of Credit if both parties agreed 

Letter of credit, Swift meathod

by using letters of Credit or bank guarantees. Some projects require sovereign guarantees