Bitwarden - Allows me to have unique, strong passwords for each of my profiles without worrying about forgetting it.
Aegis - Great open source 2FA app.
Standard Notes - Great note-taking app. I have it installed in all my devices which allows me to seamlessly switch between devices.
TrackerControl - Block all the trackers in the various apps installed in my phone.
Plees Tracker - As someone who has been trying to correct their sleep cycle, this app has been invaluable for me. It is an open source app that allows you track the duration of your sleep daily and then makes charts from it.