I put my money on NY
You are thinking of just New York City
Just go 30 Miles North of the city people are just as armed as they are in Texas, the land is just as vast and rugged (The largest park in the lower 48 is not Yellowstone, Yosemite, The Grand Canyon or the Everglades, it's the Adirondack Park in NY which is bigger than all of them combined!!!)
And if somehow you did get up here, our brutal winters will take you out.
Upstate NY has no Alamo; Saratoga, Ticonderoga, French-Indian, Lake Champlain, we won them all
I was referring to the city dwellers. The New York Army probably will be made up mostly of poorly educated and poorly motivated city people because the upstaters would not be as willing to fight for the goals of New York City. If The Unites States fell apart, the divisions would not be along just state or regional lines. I can see cities and rural areas within the same state waging their own private wars which would prevent them from thinking about forming alliances or taking on other states for several years. Even after the states began to stabilize “New York” or “Texas” would not have the same borders as before.