NYC becomes the American Montreal (i.e. #2)

[Skill and Chance]

The jumping off point would probably have to be in the 19th century.  New Orleans would be the leading candidate.  Probably requires a combination of:


a. Early industrial activities are more sulfate aerosol intensive and less carbon intensive, so the net effect of industrialization on climate is cooling and falling sea levels relative to early 1800's conditions

And:

b. earlier and stronger Catholic activism against slavery, potentially leading to abolition at the state level pre-Civil War, or at least leading to a strong enough anti-slavery minority that they stick with the Union in the Civil War like the Border States.  In fact, Union control of the mouth of the Mississippi from day 1 might preempt the Civil War entirely or turn it into something like the Whiskey Rebellion that is easily put down in a few weeks.  A free New Orleans could conceivably overtake NYC in the late 19th century.

Or, failing that:

c. Longstreet's forces resoundingly defeat the white supremacist militias in the 1870's.  Perhaps he becomes governor.  An aggressive response to the Redeemers continues and most Reconstruction-era policies are never repealed at the state level in Louisiana, even as they collapse in other states.  New Orleans absorbs much of the Southern black population in an early Great Migration, and much of the open-minded Southern white population with it.  Decades of substantially free elections make it the least corrupt and most attractive place in the South to do business, so it absorbs the early 20th century oil and gas industry and becomes a stronger and stronger trading hub.  NOLA eventually overtakes NYC during the Great Depression.