I have finally learned enough GIS to create maps from the recently released Census 2010 data. Below is a map of the population density of the City of Seattle. The map is drawn at the level of Census Block Groups (smaller than census tracts). The five most densely populated census block groups in Seattle are:

1. 80.01 Group 5 (Belltown): 110,810 ppl/sq. mi.

2. 80.01 Group 4 (Belltown): 67,489 ppl/sq. mi.

3. 74.02 Group 3 (Capitol Hill): 65,855 ppl/sq. mi.

4. 53.01 Group 1 (U District): 64,672 ppl/sq. mi.

5. 53.02 Group 1 (U District): 61,854 ppl/sq. mi.