For longer trips, it is probably less important to stay near the key tourist attractions, and probably more important to be near a good transportation hub. This starts to be a lot more subjective, and depends more on what one is looking for. Probably the best way is to compare the subway map on Naver Maps (see Using Korean transportation apps (Naver Maps, Kakao Maps)) with the key areas mentioned in Key Areas (Part 1) and Key Areas (Part 2).
As a general note, line 2 (green) is generally the most important line in Seoul, circling around the city. With the highest ridership of any subway in the network, it usually has the shortest waiting times, and may be a convenient choice to stay nearby. Just be aware that it can get very busy during rush hour (especially near the Gangnam area).
Line 2 (2호선)
