In defense of the Ritz Carlton Millenia Singapore, a hotel that I go back often to stay, I don't think the property is "very very old and aging badly". Yes, the hotel's age is old as per their name, they opened in 2000, but they have gone thru several cycles of renovation and the rooms looks freshly contemporary. Their bathrooms esp in premier suite are still the best in the business. St Regis tried few times but always service issue. Their "Butler Service" is downright annoying because everything has to go thru the butler and they are often not available when call upon. Andaz strictly speaking not exactly luxury, more like just a fun hotel to stay esp for young people. Four Seasons have renovated their rooms recently but I have not checked them out yet, but pics are already on the website. Should be worth a try.

No idea about KL or Brunei as I don't visit these cities.