Since people are mentioning the apps they like, for USA users, I can heartily recommend Topo Maps US (https://apps.apple.com/us/app/topo-maps-us/id1033846196 ht

Since people are mentioning the apps they like, for USA users, I can heartily recommend Topo Maps US (https://apps.apple.com/us/app/topo-maps-us/id1033846196 https://apps.apple.com/us/app/topo-maps-us/id1033846196) which is completely free and the work of a single developer. It's fully offline and uses USGS Topo, Imagery, and Basic Imagery. The downside is that it takes a while to download the tiles (only once though) because you do download all the tiles for the area you select. He also has a version for Canada, which I haven't tried.