You want something in Jongno. Directly in the middle of Seoul. It's also very close to Gwanghwamun, Gyeongbokgung Palace, Namdaemun Market, Gwangjang Market, the alleys and tourist area Insadong. You can make one giant loop of the area.
Try the Lotte Hotel? Or The Plaza Hotel? They both have club level. You won't get your major chain points, though. The major chains, like Intercontinental are located in the corners of Seoul, usually. That'd take time to commute from there on the subway. While Jongno is very centrally located, by subway you'd be zigzagging around to get there and see stuff from areas like Gangnam, Yeouido, etc. There are many buses that make it easier on time, but for a first time visitor, they can be very confusing.
If you only have 30 hours and want to stay near markets and sites, you need to stay in Jongno.