I use Mostly FOSS apps, I'll name few that I think are commonly useful for others.
Droid-ify - Popular Fdroid Client
Fairmail - Advanced Mail Client
Bitwarden - Password Manager , using .eu domain
Splitcat - Splitting and Merging large files 4gb+ while copying/moving
Fetchit - Download media from facebook, Insta, twitter , very straightforward app
Sharing - Share files via browser - both devices need to be on same network
GCam photos preview - view images shot on custom gcam without gphotos app installed on phone, helpful while using Degoogled ROMs
Untracker - Remove Trackers from Links before sharing
Amaze File Utilities - dozens of features , check by yourself.
Image toolbox - edit, crop , compress images
Localsend - Share Files on same network, Crossplatform . I use it to send files to my Android TV
FFShare - My Favourite, Compress images /Videos on sharing menuwith single tap
Aurora Store - Alternative Store to download Playstore apps
MuPDF Viewer - Basic and Simple PDF viewer, very snappy
Scrambled Exgif - remove exgif and metadata of image on sharing menu.
Libretorrent - Torrent Client I regularly use.
Antennapod - Open-source podcast player
Share via HTTP - can't open but , share files to others devices directly from sharemenu
Excluded some apps that already someone listed on thread