The new Lotte World Tower, currently the highest building in Seoul, offers a great viewing platform from January (2017).
Obviously, I had to see.
I don't think it will dethrone the all mighty Seoul N-Tower, however, the views from the 120th floor (486m) are pretty stunning.
Jamsil (line 2 or line 8)
KRW 27.000 pp (£19 pp) which is a pretty steep price. The N-Tower in the city centre would be, in my opinion, a better and cheaper experience.