I like Maps.me https://maps.me/ because it works offline and I can easily add markers to it, which are saved to a simple XML file on the phone (so it means even the app disappear, I can move them to another app). I also like that I can correct or add new places via my openstreetmap.com account.

It's based on OpenStreetMap and is reasonably accurate. The main disadvantage is that search is not that great but you get use to it once you know what it works and doesn't work with.