In 1780 where did the patriots suffer their worst defeat of the war?

The twin defeats at Camden, South Carolina (leading to the surrender of Charleston) were the worst defeats of the war in some ways.

The series of defeats in and around New York City early in the war nearly ended the whole revolution. Taken together, the Battle of Brooklyn Heights (or Battle of Long Island), New York City, Harlem, Fort Washington, Fort Lee, and more, left about half of Washington's men captured or killed.