What is the most important rule when designing an API?

These are some rules i apply that may not suit everyone:

• share Ideas with your team
• make sure you design user faced API
• make distinct APIs for product admins and standard users
• always design with security in mind
• design it with separation concern
• list your expected errors
• do not handle unwanted use cases because 'maybe it could be used'
• do not add complexity
• make it reviewed by another team

Make it save for ways you dont think about in the first way.

Don't make me think.

Keep it consistent. (Used patterns and naming)
Don't make the frontend do the data processing.

Do one task, and do it right, don't overthink it.

If you want a rule that's less verbal and more technical, refer to SOLID principles.

Make it functional not technical, because the underlying technic can change more often than the desired function.

Use the Open API standard with Swagger to simplify documentation and allow developers and users to work with it in a meaningful way.

Modularity. Each part has its own resposaility (easy to maintain)