There are lots of street food in Seoul. Among them I like 떡볶이 most. These days, there are some franchise 떡볶이 store across Seoul. It is actually stree food so you can find everywhere in Korea selling 떡볶이 and other delicious food. Here are my favorite 떡볶이 franchise in Korea.
Below is the link from naver blog for Addal 떡볶이.
Below is the link from naver blog for Kukde 떡볶이.