These are the twenty-one Buddhist monasteries of the Republic of Korea I have judged to be the most-worth-visiting, also among the largest and most religiously-important in the entire nation. Criteria used include contemporary size, long history of importance within the religion, national reputation, and architectural and cultural treasures -- the final judgment based on my own two decades of visiting them. 21 might seem like a strange odd number, but that's how the list came out -- to list only the top 10 would leave out some very important temples, and to make it 30 would include some that really don't rank in the same category as these 20, deficient in one or more of the criteria. The "Gu-san" or "Nine Holy Mountans" is a common concept in Korea, and so perhaps 21 is appropriate, as 19 from the dominant Jogye Order (70+% of Korean Buddhism) and one each from the two next-largest orders, Taego and Cheontae.
These 21 temples include 13 from eastern side of the peninsula (east or north of the Baekdu- daegan Mountain Range) and eight from the western side. 9 of them are located on mountains that are part of the Baekdu-daegan mainline, and the others are on Jeongmaek branches.
These monasteries are all among the top-50-or-so most interesting places to visit and most popular pilgrimage-destinations in contemporary South Korea. Most of them have many hermitages on the slopes above them, after all together serve the practices of hundreds of monks. They all contain many profound religious-cultural treasures on their grounds and in their shrines or museums. They are listed here North-to-South, not any order of relative importance; and they are named by the mountain that hosts them, in the traditional style. It is equally true that "such a great temple was established and maintained for so long at that mountain because the mountain is so holy" and that "the presence of such a great temple increases the reputation for sacredness of the mountain that hosts it". Click on the name to see my photos of and commentary on that temple, its host mountain and its San-shin shrines.