The most important thing to avoid Apple rejection is ensuring that your app complies with Apple's App Store Review Guidelines. These guidelines cover every aspect of app functionality, design, content, and performance. Here are the top priorities based on your app's type (e.g., a car rental website's app):

1. Functionality and Stability Avoid Crashes and Bugs: Test your app thoroughly across different devices, iOS versions, and scenarios. Ensure all features are working as intended (e.g., car booking, payment systems, account creation, etc.).
2. User Experience and Design Follow Apple’s Human Interface Guidelines (HIG). Keep your interface clean, intuitive, and iOS-like. Optimize for all supported screen sizes, orientations, and accessibility.
3. Privacy and Security User Data Protection: Explain why and how you collect data, and ensure you follow Apple’s App Tracking Transparency requirements. Include a clear, accurate privacy policy URL when submitting your app.
4. Content and Metadata Ensure all app descriptions, screenshots, and preview videos accurately represent your app’s functionality. Avoid placeholder content or unfinished features.
5. Payment Compliance Use Apple’s in-app purchase system for any digital goods or services sold within the app. Ensure transparency in pricing and clearly inform users of costs.
6. Device Compatibility Test compatibility on all supported iOS devices and versions. Ensure smooth performance on both iPhones and iPads.
7. Adherence to App Store Policies Avoid using private APIs or implementing features that bypass Apple’s rules (e.g., direct external payment links for services where Apple’s policies require in-app purchases).
8. Localization Ensure your app supports the languages of your target regions, especially if you’re catering to an international audience for car rentals. Focusing on user trust, polished design, and robust functionality will help ensure your app passes Apple’s review process smoothly.