South Korea, and I think I'd probably list my itinerary based on how different the destination country to USA. At a glance Asian countries feel the most different overall. Also as someone who moved to another country I can say most touristic timelines are very limited to actually properly experience a country. You can't say you've experienced a country w/o having experienced how day to day life works for a citizen, not a tourist, which's quite different. Like being familiarized w/ all of their grocery stores for example, IMO, the kind of things that take time to internalize the country.