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 

 Now you can buy degoogle phones straight out of the box. Rob also mentioned these guys. I agree that grapheneos is the best choice but generally I prefer that people start their degoogle journey with other options and move to grapheneos at a later stage
https://shop.iode.tech/ 

 Sure no disagreement here, I’m happy these options exist. However we are not the common denominator with tech knowledge. I have extensive experience with the public, you’d be shocked how many can’t tell you what phone they’re holding let alone understanding operating systems.

iOS is the best way to get easy default security into as many hands as possible. These charts from Cellebrite help illustrate my meaning
https://nostrcheck.me/media/eb73e43e76e964d13de7146de24cff1a32cafdea13dae56c4dd02804d821300f/c3a08ecc8b2239be485f2709941b0adc20f09b36a0d56670dfe8a26acd3822cc.webp
https://nostrcheck.me/media/eb73e43e76e964d13de7146de24cff1a32cafdea13dae56c4dd02804d821300f/b57eeeb37e1b34e53a63ff348bdfcfd1655baf625bb6f7edf99a795ad4985f51.webp