I'll add my recommendation on the Westin Chosun (just called the Chosun locally). I've been staying at there several times a year since 2002 and service/quality has stayed consistent on all of my visits. It is centrally located in the city - near the financial district.
The best bus service from the airport is the KAL Limosine. They have about 6 different routes - I think the Westin is #4. You can take it regardless of the airline that you fly. Buy tickets at the counter inside or on-board. As mentioned earlier it is about 50 minutes but you can hop out at the Seoul Plaza hotel and walk across the street to shave 5-10 minutes off. The other option is to take a taxi - if you do this call ahead to the hotel and have them send a car. It will be the same price and they will let you check-in in the car (about $100).
If you are Platinum, they will give you access to the executive lounge even if you are not staying on an official exec club floor. The lounges (there are 2) are excellent for breakfast and free drinks/snacks at dinner. All of the hotel restaurants are good, but very expensive. Take the 5 minute walk to Myung Dong for very good semi-casual fare.
As an aside on quality, during my stays there I've run into Will Smith and the 1st President Bush.