Wisconsin Historical Atlas- County Boundaries
Like many others, I had difficulty picking just one map. I finally decided on "County Boundaries" (pg. 70). The county level seems to be just the right level of granularity for many purposes- much finer grained than states, but graspable with some effort in a way that municipalities might not be. This is reflected in the popularity of the county level in the presentation of GIS material- statistics of many, many sorts "fit" well onto a county map. In addition, being from Wisconsin, I've found that many people identify with their home counties, and use it as shorthand for identity. I've met many people who say "I'm from (blank), a really small town that I'm sure you don't know, but it's in Marathon County."