Koguryo (China) - Step Pyramid Tomb, perhaps most important stele in East Asia outlining the history of Goguyeo, exquisite tomb murals.
If we can only vote here:
Kyoto - worthy collection of sites representing Japanese Imperial Capital
Nara - worth collection sites that represent the formation of the Japanese state
3 Korean Sites (One Silla, One Baekje, One Goguryeo)
Maybe, though not all 3.
Gyeongju - Massive Tombs, Astronomical Observatory, and sacred mountain filled with carved buddhas and several pagodas.
Baekje - Because of Mireuksa Temple and King Muryeong's Tomb, also represents the spread of Buddhism from Baekje to Japan.
I personally find the Goguryeo site in China the most interesting and possibly most (or of equal) significance, but I have never been there. Regarding the Baekje and Gyeongju WHS, either these 2 sites cancel each other out, or we have to go for one. I know many people who frequent this website have been to Gyeongju, some have been somewhat disappointed. However, few of us have fully visited the site (including the sacred mountain area), unfortunately I never had the chance to make the hike. Few of us have been to the Baekje sites since it was only recently inscribed, I was fortunate enough to visit in 2011 7/8 of the serial sites. Therefore, I feel I actually know the Baekje WHS better than the Gyeongju WHS. King Muryeong's Tomb is hugely important and one of the greatest archeological discoveries on the Korean Peninsula. Found in the perfectly intact tomb were items from much of East and South-East Asia. In addition, Mireuksa Temple is hugely important as the oldest and largest Stone Pagoda in the Korean Peninsula, and certainly one of the largest in all of East Asia. Gyeongju has a unique astronomical observatory, massive tombs, representative brick/stone pagodas, and a sacred mountain filled with Buddhist reliefs carved in stone.