According to the 2000 Census, Guttenberg, Union City, West New York, and Hoboken, in that order, are the top 4 most densely populated incorporated places in the country.
New York City falls in at #5. Which is much more impressive, as all these other places range between .19 and 1.28 square miles and NYC is over 300.
Be interesting to see how that all changes in next year's census.
http://en.wikipedia.org/wiki/List_of_United_States_cities_by_population_density