I'd say finding your way back would be #1.

That's different than map skills. A map is good if you have one, but not so good if you leave it somewhere and forget it.

The trick to this is reading and remembering the terrain you've covered, and looking for and remembering landmarks.

Fallen trees, rock formations, creeks, canyons and distant views of peaks and ridges and valleys, man made items (trash). Pay attention to these things as you hike in, and look for them when hiking out.

These skills are easy to practice and learn by starting with short hikes and bushwhacks (off trail hikes) and increasing your distance as you become better and more confident. You can hone your map skills at the same time.