In Spearfish Canyon, one of the rare deciduous ecozones in the Hills:
The path that follows Rapid Creek, up behind Silver City. One of the most peaceful places I can think of - quiet, high canyon walls, birches and spruce, the lovely stream.
Another lovely path, this one behind Camp Judson, not far from Mt. Rushmore. It follows a tiny stream, with lots of big granite boulders and patches of ferns.
Lastly, a dirt track out on the prairie near the Badlands.
What paths are you following these days?