Gaia GPS is still my go to after all these years (Post from 2012) It has only gotten better with time. My phone is my primary GPS for just about everything, I do have a Garmin Fenix 5X+ but normally use it as a adjunct to my iPhone or iPad.

Avenza Maps is another phone/tablet based mapping app that is improving consistently as well. Not quite as versatile as GAIA GPS; but it has some additional features that make it a good option for some applications and it has become the standard mapping app for wildland firefighting. On most wildland fires I use the two app simultaneously

I've used Avenza maps. It has access to many USGS maps that you download and then it plots your position and tracks. https://www.avenza.com/avenza-maps/