Just my thoughts on Marriott hotels in Seoul over the past 3 years. Completed 15 stays, average stay was 8 days.
My ranking weighs everything: services, cost, lobby, suite room size, bathroom, buffet breakfast, lounge, convenience, noise level, comfort, luxury, fitness center, sauna, pool, coin laundry, and restaurants.
Each category was awarded 1 - 5 points except cost which was on a 10 point scale. The result surprised myself

JW Marriot Dongdemun 61.50
Josun Palace 60.00
JW Marriot Gangnam 58.50
Courtyard Marriot Yongdungpo 42.50
Four Points by Sheraton Gangnam 41.50
The Plaza Seoul Jung-gu 41.00

JW Marriot Dongdemun and Josun Palace are unquestionably the best hotels in Seoul but for different reasons.

The Josun Palace is absolute luxury. At of all the hotels it is the most expensive but you feel that you are getting what you paid for. The only thing that it lacks is an executive lounge which costs it quite a few points and also it is fairly inconvenient to walk to the subways.
JW Marriot Dongdemun is not as luxurious. But what gives it the edge over the Josun Palace is that it does have a great executive lounge, it is conveniently situated right on top a subway station, and it is a great deal for the price.

As for the bottom hotels I will say it brief
JW Marriot Gangnam - expensive and nowhere worth what they charge. Also disliked their lounge and fitness center
Courtyard Marriot Yongdungpo - its inconvenient to get around the city from here. On a budget I would choose again though.
Four Points by Sheraton Gangnam - no lounge, barely a fitness, location itself is ok but getting to anywhere is a walk
The Plaza Hotel - never staying here ever again. Expensive and the suite rooms are terribly laid out and small not to mention outdated. What saves it is the excellent lounge and location.