AndOTP : A good app for OTP that actually let’s you backup your keys and move them easily and securely to a new phone
Blokada : A system wide adblocker for Android. 4000000 ads blocked so far on my phone
YouTube Vanced: Not really a “hidden” gem but a cool app nonetheless. I wish people knew you could get an ad free YouTube experience on mobile. It also includes sponsor block so you really get straight to content. Also let’s you get a YT popup player and shutdown the screen with the video still playing.
F-Droid an Android open source app market. Often includes apps that are not allowed on Google market.
A few of mine :