I have two main GPS apps on my iPhone currently, GPS Kit app and Gaia GPS app.
I've primarily used GPS Kit and have found it great for backcountry skiing, hiking and camping in the Catskill Mountains and Adirondacks. It does take a little while to understand how to use the app and input GPS coordinates correctly, but once you get familiar with it, it works really well offline without a cellular signal. The downside of GPS Kit, is that they seem to have stopped updating the app.
I also have the Gaia GPS app, which is really popular with a lot of functionality. I just haven't used it that much, but may switch to using Gaia GPS in the future. Gaia GPS does get lots of good reviews, but is considerably more than GPS Kit.
The one thing I really like about the GPS Kit app, is that the map Arrow Pointer will point in the direction that you point your iPhone (when it's flat). The Arrow Pointer is very responsive to the direction you point your iPhone. In the past, Gaia GPS arrow pointer wasn't as good for this, but I think they've since fixed the problem. I've found this pointer can be extremely handy to quickly know what direction you want to head in relation to the map overlay.