Answer:
Eastern USA had the largest.
Here is the statistics for 1860:
Rank City State Population
1 New York New York 813,669
2 Philadelphia Penn 565,529
3 Brooklyn New York 266,661
4 Baltimore Maryland 212,418
5 Boston Mass 177,840
6 New Orleans Louisiana 168,675
7 Cincinnati Ohio 161,044
8 St. Louis Missouri 160,773
9 Chicago Illinois 112,172
10 Buffalo New York 81,129