Hands down the most advanced and secure smartphone os is grapheneOS.
It only runs on pixel devices. App sandboxing, buffer overflow protection, separate users, optional sandboxed google play if you need it for banking apps, automatic reboot after a set time to re encrypt your filesystem, assigning permissions for app access to folders and the list goes on and on.
Aperture Science. We do what we must, because we can. For the good of all of us. Except the ones who are dead.