These 12 famous mountains (and groups of mountains) are the ones I have judged to be the holiest and most-popular pilgrimage-destinations in contemporary South Korea (the Republic of Korea), mainland only (not the islands) by the criteria explained on this page and my 20 years of visiting them and studying this subject. They break down in a convenient and balanced split between five in the central and western regions and five in the eastern regions (mostly, the Taebaek Range and its southern extension now dubbed the "Yeongnam Alps", going as far west in the southern region as the Nakdong River). They are listed here North-to-South. Click on the name to see my photos and discussion of that mountain and its San-shin shrines.
Out of these 12, Jiri-san is supremely sacred, and the next 3 are (in no order) Samgak-san, Gyeryong-san and Taebaek-san
Central & Western Korea:
The Bukhan-sanSub-Range 북한산 especially Samgak-san 삼각산 840m and Inwang-san 인왕산 388m, and also including An-san, Bugak-san, Dobong-san, Sapae-san, Surak-san, Bulam-san and Acha-san. North and East of Seoul City. Some of these are in the Bukhan-san National Park, others are Local Parks.
Sogni-san속리산 North Chungcheong Prov, National Park, 1058m
Gyeryong-san계룡산 Daejeon & Gongju Cities, National Park, 845m
Ma-i-san마이산 North Jeolla Province, Jinan County, Provincial Park, 618m
Jiri-san지리산 Gurye County of South Jeolla Province; Hadong, Sancheong and Hamyang Counties of South Gyeongsang Province; and Namwon City of North Jeolla Province; National Park, Cheonhwang-bong 1915m, Banya-bong 1733m & Nogo-dan 1507m