The 2 apps I use the most for hiking...
1. Topo Maps
This lets you download usgs quads to your phone. That means you have the quad map stored locally and don't need a data or cell connection of any kind to use the map. The added handy feature is that you can use GPS and it will overlay your gps position on the map so you can see exactly where you are. Again, USGS quads and no cell service needed. It's a great app.
2. Trails
This app is really meant to be a "tracking" app that breadcrumb trails you around as you go, but I don't use it for that. It also has the ability to "import" a gpx file and it will overlay that route onto the map. You do need a data connection for it to display the map, but it will cache that display so you can access it later without a data connection. The other handy feature is that it will display your gps location as well so you can see if you're "on the trail" or not. I actually used this when I did Whitney last year as it was my first time going up in complete darkness and I had read about how the trail could be hard to follow at times. This little app was invaluable as I checked it many, many, many times as I went
