My go to for a GPS/Topo mapping app for iOS is Gaia GPS. The free version is good, but an upgrade to premium turns it into the best mapping app I have ever used. There are a ton of different map options that can all be downloaded for offline use. Maps can be layered so you can build a map set specific for your needs. Since I found GAIA I no longer use a stand-alone handheld GPS.

Second to GAIA is Avenza Maps. It doesn't have all the features of the Premium version of GAIA, but if you are just looking for a straight forward GPS program with offline topo Avenza might be what your looking for.