The one built into the OS. You shouldn't trust another app with full photo access.
Huh? Are there other gallery apps to iOS other than Apple's app? /s
If you want to choose between multiple gallery apps, Android would probably be your best choice. Fossify Gallery is the best one if you ask me.
Plus, any device from Apple is not a good privacy choice. Just saying. You are the product - literally. You don't even own the devices even if you buy them! Apple has that much control over you!
Update: I checked the App Store on my "I want to learn the system" iPhone that I got for free. I did found numerous gallery apps, but none with as good privacy features as what Fossify Gallery have.
Viewer? Probably just Quick Look. It's built into the Files app.
If you want to edit, just use Apple Photos. Everything is in there anyway.
Privacy has become a very important issue in modern society, with companies and governments constantly abusing their power, more and more people are waking up to the importance of digital privacy.
In this community everyone is welcome to post links and discuss topics related to privacy.
much thanks to @gary_host_laptop for the logo design :)