We all know open source like graphene is top tier for hardcore privacy. However the general public will never learn or install alternative operating systems. Never gonna happen. They buy a phone and use it as is. 

The real choices for 90% of the public are manufacturer customized Android, or iOS. I will always recommend iOS between the two, it’s far more secure. Ask cellebrite or the NSO group. If you’re an “at risk” individual, turn on lockdown mode and advanced data protection. Regardless of jurisdiction, nobody can decrypt your data.

All comes down to knowing about the tools