If it's a special occasion and not concern about price, i think about the Oriental, Shangri-la, and Peninsula. All are hotels by the river, for me not actually in the centre of all things like Sukhumvit, Silom, Siam Square areas but not far from skytrain station, shops, departmentstore, restaurants, etc. Just walk few minutes.
However, if best location to stay, for tourists, I still recommend Sukhumvit, Silom and Siam Square. So good hotels on these areas with same rating with Banyan tree should be Sheraton Grande, the Landmark, the Inter-Continental, Grand Hyatt Erawan on Sukhumvit area, and Dusit Thani on Silom.