There are also Esri maps: https://developers.arcgis.com/en/features/mapping/

...if only there were a practical Android app available.

It's not free but Gaia GPS works well on my devices. I believe they recently released an android client. Besides the various Topo maps they have an osm layer that's available via internet or for offload cache.

I use it for when going to a rural area I'm not familiar with, since Internet and phone signals is still non-existent or very slow in those areas.