A lot of the field staff where I work use Avenza Maps. We use the pair "Pro" version, but the free version is still excellent.  It allows a fully offline map interface, and it appears that all of the USGS topo quads and a good number of National Parks maps are available for free.  witht he paid version, we can also upload our own PDF maps, as well as record and share GPS points on any map to share with other users.