GrapheneOS

GrapheneOS is the best choice when it comes to privacy and security. GrapheneOS is a Privacy and Security-focused mobile OS with Android app compatibility developed as a non-profit open source project. It’s focused on the research and development of privacy and security technology, including substantial improvements to sandboxing, exploit mitigations, and the permission model. It recently added support for sandboxed play services.

Requirements: Google Pixel device (latest models recommended)

If you can’t get a pixel then avoid using ROMs that require you to keep the bootloader unlocked and stick to your stock ROM.