I would guess northern rockies (due to low demand for roads, and limited places to put them)...a secondary valley over a line of peaks from a main valley, on a north facing edge, with an average elevation above the treeline. Basically, montana, or northwestern north dakota.
However, the actual answer is Nevada, specifically, NTS. Given the size (>1300 square miles, roughly 90x150 miles), zero full-time population, huge areas of "lethal force authorized" desolate plains with no active projects, I would bet you could easily find a location there where 24/7/365 the nearest human being would be >20 miles away.