Having an offline, updated map in my pocket is invaluable when hiking and cycling in the mountains. I have been contributing to it as much as I can, to at least give something back.
The answer there likely dives too much into personal preferences to be a "one size fits all" answer.
Both are available on f-droid, so just try both and pick the one you prefer:
Osmand: https://f-droid.org/en/packages/net.osmand.plus/
Organic Maps: https://f-droid.org/en/packages/app.organicmaps/
Organic Maps is a better starting point IMHO.
Do you know of an OSM app that can do this is offline mode?