Gaia GPS, one of the best available smart phone apps for wilderness navigation, has an amazing bounty of map layers. It's easy to get a little overwhelmed and wonder which ones might be best for a particular activity. If you're fairly new to Gaia, you may want to check out the four listed below. No doubt you will come up with your own favorites over time, but these layers will be a great start.
Driving urban areas and paved roads: MapBox Streets HD
Major roads are clearly delineated, easy to read at a glance while you’re driving, pretty good points of interest. Nice road network, but no elevations. Minor roads can be a little hard to see. See example below left for San Francisco.
Hiking on trails and driving obscure backroads: Open Cycle
Small roads have the same line weight as bigger ones, so it’s easy to see when you’re at a tiny junction. Open source maps means map features are likely going to be more accurate than other sources. Note the bicycle path in the valley floor in blue, but also all of the hiking trails are shown, in red dashes. Contours and shaded relief are shown, but no elevations. (The resolution of my screen grab below for some reason looks a little fuzzy, it is quite a bit better in real life.) See the example for Yosemite valley below right.
Satellite view: Satellite with labels
Gaia GPS has several different satellite layers, and “Satellite with label” is my favorite. It seems to have the clearest imagery, especially when you zoom way in, and having roads labeled is a big help. (They recently have added a hydrology layer, which shows every little tiny stream at a certain zoom level, which is kind of distracting.) See the Olympic Peninsula example below on the left.
Backcountry hiking and climbing: Open Topo
Great shaded relief, shows some general terrain such as forest, rock, and snow, has lots of streams and drainages (sometimes too many, IMHO), contours and elevation (in meters). See the Mount Olympus example below on the right. (The purple line is a GPX track of the main climbing route overlaid on to the map.)
Open Topo is no longer part of Gaia GPS, so you need to add it as a custom layer (which, like most things in life, is easy once you know how.) Learn to add Open Topo as a custom Gaia map layer here.