The best ginseng soup is found at Exit 2 of Gyeungbokgung station, take a left after GS( about 3-5 mins walk). The name of the restaurant is Tosokchon.
There is no english menu available but it's not a big problem for foreigners. Everyone goes there for Samgyetang. So just point at the soup stone pot at the next table. Make sure it's a white chicken because they have Black Chicken Ginseng soup too!
A ginseng chicken soup costs around 11,000 won. The chicken is stuffed with rice, chestnut, ginseng and jujube. The soup is rather bland, so you would have to add your own salt at the table.
The restaurant also provides a few side dishes - kim chee, pickled radish, raw garlic and chilli paste - and small metal bucket at the side of the table. I didn't know what the bucket was for until the kind waitress explained to us to discard the chicken bones in it.
Also, everyone at the table gets a small cup of cold ginseng wine for free too!