Geography of government benefits
I missed this one a while back, but The New York Times had a look at the growth of government benefit programs, such as Medicare, Medicaid and Social Security, in the United States. On the surface, it looks like your standard choropleth map that shows percent of income from government benefits, but there's a lot going on here that makes the piece really good.
First, the arrows on the top right let you browse through decades, going back to 1969. Roll over counties to see a time series for the corresponding region against the national average. The sidebar on the left lets you view breakdowns for different programs. And finally, the guide to key trends provides a narrative for noteworthy regions and patterns.
Now that's some good data journalism.