From my research, these are decent business hotels. Cheap (USD$90, JPY10k, KRW 120k). Good location to famous places:
I stayed at GLAD Mapo and GLAD LIVE. Because I had this struggling that if GLAD Mapo is so good. Then I switched to a worse one. That will ruin the rest of my trip because I kept thinking about how good it was.
They are good business hotels. I saw Shilla Mapo is literally 2 minute walk from Glad Mapo. They are both within 3 minutes walking of Gongdeok station.
If you stay there, do not miss the Fritz coffee company, Dohwa. It's a must go!
I decided to stay in Gangnam area because I want to try what‘s like on the other side of Seoul. This area is far from touristic attractions so I recommend you try this after you are bored with all touristic stuffs.
For more enjoyable staying:
I have seen someone asked on Twitter and all replies were Park Hyatt Seoul.
Very first 5 star hotel which to be the go to host for dignitaries. Also has a lot of facilities and great food I heard...
Hope this is useful.