From what I can tell, most of the key nuclear development sites in Iran are clustered either within or slightly north of the city of Isfahan (e.g., Natanz and Fordo): https://en.wikipedia.org/wiki/Nuclear_program_of_Iran

I forget who, but one of the foreign policy guests on Sam Harris's podcast emphasized how easy it would be for the U.S. to shutdown Iranian oil exports by bombing a single port. I forget which port he identified in that podcast, but after some googling, I suspect it was Kharg Island. But my search also brought me to this analysis by AEI identifying potential candidates for U.S. military strikes in Iran, and it looks like it might make more sense strategically for the U.S. to nuke air and naval bases around the Strait of Hormuz, which defend almost all of their oil exports. Bandar Abbass is the most likely city to be nuked in that scenario: https://www.aei.org/op-eds/military-strikes-on-iran-here-are-5-possible-targets/

I'm guessing the U.S. is slightly more likely to nuke nuclear sites (as opposed to oil shipping ports) in Iran because it renders them permanently unusable - something the U.S. wants - whereas the U.S. isn't particularly invested in pointlessly destroying the Iranian economy and sending the global price of oil through the roof.

TL;DR: If you think the U.S. is likely to nuke Iran, then I think Isfahan is the most likely city to be nuked.

@SteveMichaels the US will not nuke Iran but Isreal might.