9. New Orleans, Louisiana 1915 New Orleans, Louisiana September 1915 Death toll: 275
This Category 4 storm caused Lake Pontchartrain to overflow its banks, killing 275 people. That scenario is one that hurricane experts don't like to ponder because if the city, surrounded on three sides by water, is hit by a major hurricane, the storm surge might inundate the city.
That phrase "don't like to ponder" resonates like a cracked bell today. This is one case where pessimism would have been preferable to hope.