Three national monuments in Arizona are among the least visited in the U.S. as described in a story in the LA Times:
#20 Chiricahua
#19 Tonto
#17 Pipe Spring
The least visited monument is Aniakchak, Alaska. Quite a few of the 20 are in New Mexico. [right, Dos Cabezas Mtns, Chiricahua Natl Mon. Credit, Natl Park Service]
Thanks to Geology.com for spotting this story.