DAVx & ICSx (nextcloud contacts and calendar sync)
DNS66
Jellyfin
Spotube
From FDroid (really droidify from various repositories)
FUTO Voice Input
Breezy Weather
K-9
Libera Reader
FFUpdater
Joplin
The list is massive and I’m on mobile and hate tiny keyboards. I can finish the list later if you/y’all are interested. The only thing that I actively use that is not FOSS on my phone is Google Messages, which I guess is a bit hypocritical, but its too good. Just the ability to react to messages makes it worth it for me.
I also highly recommend Grayjay. It is the best (IMHO) YouTube replacement. It is cross service, like I have odysee, nebula, youtube, and various peertube instances added as sources.
As far as what apps to watch out for, someone mentioned Simple Mobile Tools. Otherwise, I would stay away from apps that are not being updated anymore or are otherwise way too old.