Maplets - I use it all the time.

Whenever I travel to a new area, I always check Maplets and download all applicable maps ahead of time. What is remarkable is what a vast selection of maps are available within the Maplets app. I also love that they keep the maps updated, and let you know when an update is available.

Kern Canyon Camper about Maplets – The Offline Maps App, v3.8.2