Scout by Telenav seems to be well thought of, I suspect that's the best drop in replacement for Google Maps on iPhone:
But I'd say OSM mobile apps tend to be a bit more focused (or bitty) than Google Maps. So, for instance:
Maps.me - concentrates on offline viewing, target market seems to be tourists, but probably useful in general.
Komoot - concentrates on hiking/biking, planning routes, giving back surfaces of trails, elevation pathways, etc.