Four states have been former republics. Several have sporadic or persistent secession movements.
Which state, if it left the US, would make the most functional new country?
Most functional ?
Well that leaves out the Peoples Republic of Mexifornia.
The Republic of TEXAS.
They already have oil and gas and refineries in place.
They have a deep water port and a highway and rail/road infrastructure in place.
The cattle and cotton production is enough to feed and cloth the people of the state.
A large standing army wouldn't be necessary because Texas has enough civilians who own guns and know how to use them so Texas can go back to America's roots of a citizen soldier.
It's already a right to carry state.
In Texas you are already allowed to use deadly force to protect yourself, family, home and property.
The Texas Rangers were highly successfully securing the Rio Grande from illegal aliens and smugglers before the U.S. government thought they could do a better job. Not.
Even the Mexicans in Texas speak English.
Texas penal system is already in place and it's not PC and they can put a convicted murder to death faster than any other state.
The most important thing of any Republic is nationalism. Texans are proud of being Texans.
Texans love contact sports and that's important so you don't end up with a state of pantywaist liberals.
Texas already has two NFL teams and that's important and also a professional baseball team.
The Republic of Texas has already been recognized by many of the major countries in the world back during the late 1830's and early 1840's. So that problem of being recognized is already taken care of.
And there aren't that many loony socialist like Shila Jackson Lee that need to be rounded up and tarred and feathered and exiled from the Great Republic of Texas.