I have been to all but one of the UNESCO sites in South Korea. This is my favorite UNESCO site. And not just in Korea, but in the world.
Rome would be tied but Gyeongju represents (to me) what the World Heritage Site status should be doing - bringing incredible sites people otherwise wouldn't hear about into the foreground. Most people who do not live in Korea have never heard of it, and those who visit Korea typically stick to Seoul or Busan. Korea, perhaps surprisingly in an age with Korean music, food, cinema, and fashion sweeping the globe, doesn't advertise most of its amazing sites. The Jeju Lava Tubes are unlike anything I've seen before, and the dolmen sites at Gochang seem to go on forever.
Gyeongju is ripe for the kind of mystique and reverence we place on pyramids - not because they're as old or as complicated to build, but because there's something enchanting about it. Six kings and queens beneath massive mounds of slumbering earth, each with a towering crown of golden antlers. A treasure trove of beautiful artwork and wealth, items from around the world finding themselves here.
And yet the people of Gyeongju can sometimes live stunningly close to these ancient mounds. Enjoying daily life, having picnics. But the UNESCO site doesn't stop at the mounds. It includes the oldest observatory in the world, palaces, temples, monuments - all scattered throughout the countryside. There is still a ton of excavation going on and new finds are being unearthed almost everyday. Recently, there has been evidence that the Shilla practiced human sacrifice just like the Gaya (who lived further south in a smaller territory).
Gyeongju itself is well set up as an open museum. There's a sort of loop you can take that I detail in one of my posts on Gyeongju. For context, the Gyeongju National Museum is possibly one of my top five museums in the world as it has great displays and information. My one gripe is that many of the further away sites are very difficult to reach without a car. I am consistently shocked that Korea doesn't have tour buses or shuttles around to the sites. Even a clearer way to get to them would be much appreciated, since Google Maps doesn't work in Korea. Additionally, Wolseong Palace is best seen at night when they illuminate it (otherwise it's fairly disappointing compared to the palaces elsewhere in Korea).
I highly recommend this UNESCO site. I'm not sure I would recommend the nearby Bulguksa or Seokguram as I found the stunningly disappointing (you can barely see Seokguram through the plexiglass). But if you're in the area, it's fine extension. I hope more people get to see and hear about Gyeongju!