I usually direct people to Comaps, because I find it simple and very close to the mobile Google maps experience. Most people won't need to understand how OpenStreetMaps works in order to use it
But there are a lot of other apps that display the same data. Feel free to ask for more recommendations :)
There's a good discussion below about another app OSMand
(this comment was edited, original below for context)
Comaps specifically, since people will get confused if directed to the OSM project as a whole