You didn't mention whether this was UK only, but I use
UKMap which downloads 1:25000/1:10000 OS and OSM data. It's the free OS data so doesn't feature the full detail of the 1:25000 Explorer maps (footpaths etc) but it's genuinely very good. The app is paid-for (if I recall £5.99), the maps free.
You download map data as you need. Very simple, you simply select the area you need. Once downloaded, it will persist on your device. The downside is that map data is big, so you really need to download in advance over a broadband connection. Also there is no means to save downloaded map data 'offline' so it's not taking up space on your device, though you can delete and download as needed. Not a biggie, but it would be handy.
Excellent app really, very detailed mapping, works perfectly with GPS, does 3D contour mapping etc.