Heres my tier list of open source android apps, I have only included ones i feel I have used enouph to understand.

Please tell me any of your favorite open source apps as I would love to hear.

S tier- OpenCalc, firefox, thunder, organic maps, Aegis authenticator, wikipedia, translate you, F-Droid.

A tier: open camera, fossify suite, k-9, termux, simplenote, Acode, pie launcher,

B tier: gallery (by iacoblonut), peer tube, simple keyboard, floris board (might be higher if it used haptic feedback interface by default), unexpected keyboard (would be S tier if shift did not carry on selecting text once released).

C tier: FreeDcam, libre camera, geo notes, free paint.