Step 1: Read the question.
Which is the most important source of International Law in modern times?
Step 2: Recall the sources.
Article 38 of the ICJ Statute lists treaties, custom, general principles of law, and judicial decisions and writings.
Step 3: Compare the main sources.
Custom is old and unwritten, based on long state practice. Treaties are written agreements that clearly state the obligations of each state.
Step 4: Decide which is most important now.
Today treaties are seen as the most precise and widely accepted source because they create clear, binding written rules.
Step 5: Eliminate the others.
Judicial decisions and general principles are only supporting sources, not the primary one.
Step 6: Conclude.
The most important source is treaties.
Answer: Treaties