Some of them are not underrated, but I'd just share my favorites...

• AntennaPod for podcasts.
• Aurora Store if you need some Google apps to be updated.
• Binary Eye is a nice and slim qr code scanner and generator.
• CriticalMaps is a good tool if you're into critical mass events.
• GadgetBridge can be a companion app for your smartwatch (if you have the right one).
• GPS logger just logs your position in an interval you can set. I use it to track my movement in 30min bits.
• GymRoutines is a privacy friendly gym exercise app with a database of your progress.
• K9 mail (soon to be rebranded as Thunderbird mobile) is a powerful mail app.
• KeePassDX is an android password manager compatible with keepass files.
• LibreTorrent is a nice way to torrent your favorite linux distros while on the go.
• Location Map Viewer I use with the gpx files I get from GPS logger to visualize my movements.
• mpv just plays a lot of media without being a bloated thing like vlc. I use mpv for videos, vlc for music.
• NewPipeSponsorblock is available in a custom repo only.
• Ning is a nice way to get an overview over a local network.
• OpenTracks can be used in combination with GadgetBridge to record your sports with gps.
• OSMand gives you a frontend for using openstreetmap with offline maps, navigation and more.
• PDF Doc Scan is my way to digitalize important letters as a pdf on the go.
• StreetComplete is a gamificated way to fill in missing things in openstreetmap. it's fun.
• Syncthing-Fork (important to use the fork) is a file sync tool I use to backup a lot of my phones folders to a pc.
• Tusky is my favorite way to browse mastodon.
• VLC is a powerful media player (I then use only to shuffle through my mp3s).