+1. I was very impressed with the Sheraton D-Cube. A worthy contender, and better value than Westin Chosun, especially now that Starwood sneakily increased 'peak' periods 10-fold and the Westin seems to have 'peak' dates more often than not.

The two hotels you can safely forget about -because of location, not because they're bad hotels- are the Sheraton and W Walker Hill. Far out and really suboptimal for tourists.

I would also advise against the PH (along with the ICs and JW) because of location. A worthy option to consider (since you asked about Hyatts), is the GH, which has an excellent location, great view, nice lounge, and easily the best 'resort-like' relaxation opportunities in Seoul.