Review: Organic Maps
Organic maps is FOSS and uses Open Street Map data. You can download regional maps for offline usage. All the basic mapping features are there, though some more advanced ones might still be lacking at this date.
Compared to Google Maps and the like, Organic maps respects your privacy.
Compared OsmAnd — which is the de facto mobile app for OSM — Organic is faster and has a cleaner UI. OsmAnd gets quite laggy when there are a lot of maps downloaded.