I haven't stayed at the Marina Mandarin for a while but their refurbished rooms look quite nice. Not quite the luxury hotel, imo.

The FS is nice but I don't quite like the decor there. Not sure if refurbishments are going on/completed.

I think the RC has the rooms with the 'wow' factor. If you get to choose, the Marina Bay view is better than the Kallang one. The service was pretty poor (snotty check-in agent, messed up bookings, sloppy housekeeping) though and I haven't been back since. Doesn't help that they didn't respond to written feedback.

I think the Shangri-La Valley Wing is the best choice for luxury in Singapore at the moment. I'm not sure if the OP can get that though since Valley Wing is like a Club floor.

If you want a post-colonial Singapore experience, you could give the Fullerton or the Raffles a try.