It is a bit late for this reply, but I thought it might help people who are looking for answers

The best 3 I could find after a long time search are:
OSMAND: Very well designed, good interface, easy to use, full of features, and stable. Although the latest update made it a bit sluggish and unstable. You also have the limit of 10 map downloads (and for those who live in Canada or US, the provinces and states are in a separate file each, so for example if you want the whole canada map you need to download all your 10 free maps!). The bad thing is, every time the app downloads the updated version of the already installed maps, your free map counter counts this as one download! So by every update you get closer to your limit! (somebody correct me if I am wrong).
Map Factor Navigator: This is also a good app. Uses the same maps from Open Street Map. A good interface, simple to use, and stable. Not as many customization options as OSMAND, but still gets the job done. It is completely free, and you download each country in one map.
NavFree: This one is probably the most known of the three. It is completely free. However, I don't recommend it since it is unstable, and every now and then it pops an error that says your map file is corrupted and asks for permission to download it again (this happened to me on several devices). Also IMO the interface is not as clear as Map Factor Navigator.