It’s the Salt Lake City metro area.
In this post, W. Bradford Wilcox points out a recent study by Harvard and UC-Berkeley:
[O]ut of the largest 100 metropolitan regions in the country, the Salt Lake City area is best at promoting absolute economic mobility for lower-income children and embodying the Horatio Alger story.
What accounts for the area’s success? The study does not specifically focus on Salt Lake’s comparative advantage for kids, but it does suggest two factors that are key to fostering income mobility for children around the United States: family and civil society. Specifically, the Harvard-Berkeley study that the New York Times called the “most detailed portrait of income mobility in the United States” found that the most powerful (negative) correlate of such mobility was the share of single moms in a region. This means that children were most likely to realize the American dream when they came from regions—like the Salt Lake City area—with comparatively strong families.