Article 38(1) of the Statute of the International Court of Justice Defines 5 recognized sources of International Law:

•  International conventions (e.g. Treaties)
• Customary International Law (General Practice of States and International Bode)
• General Principles of Law (Recognized By Nations)
• Judicial Decisions (Both International and National Courts)
• Writings/Teachings of International Scholars (Secondary Resources)

Soft law is another recognized source of international law. It  includes non-binding principles, agreements and declarations that are not legally binding. 

Soft law instruments include resolutions of international institutions, such as United Nations General Assembly Resolutions, or policy of instruments of the European Union such as 'Recommendations', 'Guidelines", "Communications", etc.   

No single source of international law is considered inherently more important than the others. The hierarchy of importance depends on the specific context of a legal issue.

Treaties: When a treaty exists that addresses a particular issue, it is usually the primary source because it directly binds the parties involved. Treaties often take precedence in disputes where states have explicitly agreed to be bound to certain provisions.