My top favorite apps from f-droid are (no order)
camera roll: a very low space usage video player and a image gallery with no internet permissions.
batch uninstaller: is an app that makes it quick and easy to uninstall a number of apps
superfreezz: A greenify clone but open source. Freezes apps, and some system apps. If
you freeze system apps be carefull what you mess with.
open camera Good camera, no internet permissions.