falcon9xr

So the most recent trip I stayed at the following properties in Seoul, all on points as a Platinum Premier:
JW Marriott Dongdaemun
The Plaza Seoul
Sheraton D Cube City
(Sheraton Grand Incheon) (this isn't technically Seoul, but it is really close to the largest airport servicing Seoul, so mentioning it)

I also stayed at the Sheraton Seoul Palace Gangnam on a previous trip on points as well (this was as an SPG Gold).

My rankings would be as follows:
1) The Plaza Seoul - My main concern against this hotel was the lack of hot dishes at the evening happy hour, but @gora14 mentioned that I may have missed them, which I think is true. Otherwise, the location is great, the sauna/fitness area is top-notch, though you do have to move to a different building, and the service is excellent (it's always nice to receive a bottle of wine in the room). The breakfast is not as good as in the restaurant, but I'm not picky about that. You'll find a cab very easily when leaving the hotel, and there are a TON of restaurants and convenience stores and such around if you're looking for something. It also has direct access to the subway, which is convenient. One downside is that there are protests/demonstrators occasionally. You probably won't be accosted by them going out and about, but it'll be loud.
2a) Sheraton Grand Incheon - So this isn't technically in Seoul, but if you don't mind being away from the action, or want to be closer to the airport, I would pick this property. It doesn't have the feel of being an "airport hotel", and I think Incheon as a city has it's own flavor that is distinct from Seoul proper. Also Seoul is accessible via public transportation from Incheon. I think it also helps that I was treated very well as a Platinum Premier guest.
2b) Sheraton Seoul Palace Gangnam - I didn't stay here on the most recent trip, but I intend on staying here during my next trip to Korea. It has a prime location, as it is diagonally across from the Express Bus Terminal, which has access to three subway lines, as well as buses that leave Seoul. The service here is great as well, and the breakfast and evening happy hour offerings in the lounge are excellent. The fitness center/spa is top-notch as well. I do remember the room being a bit small, so there's that.
3) Sheraton D Cube City - Definitely a property away from the action, but is nice in that it is connected directly to the Hyundai D Cube shopping complex, as well as the subway. As this is Seoul, there is still a ton of stuff available as far as restaurants and shops and such, but it definitely is away from the tourist locations. So that can be a plus or a minus depending on what you're looking for. Service is great, and evening offerings in the lounge were great as well. Like the other Seoul hotels, the fitness center/spa is really nice. Cabs seem to have trouble finding the entrance to the hotel (the front door is through some side roads that are a bit confusing), so just something to note. Probably won't stay here again, just because the location is away from the stuff I'm interested in.
4) JW Marriott Dongdaemun - This may be a situation of mismatched expectations, but I was hoping for more from a JWM. I definitely wasn't expecting to have to pay for use of the sauna as a Platinum Premier (to be clear, ALL the other properties I stayed at included sauna access). The location was a bit of a bust as well. I thought that Dongdaemun would have some more along the lines of touristy things to do, but alas it is literally just the gate (which you can't walk through). The Design Plaza was definitely overhyped from what I expected, and so was the Cheonggyecheon stream. A good spot if you're looking to do a bunch of clothes shopping late at night, but I think that's a rather niche group. Definitely not a good use of points, and I think they are actually going up a category in March, so it becomes an even worse value. Definitely never staying here again.

Hope that helps!