In the Silom area, I would suggest the only 2 hotels that fall into the luxury class are The Oriental and The Shangri La (particularly the Krungthep Wing). Neither of these are on Silom road but are not to far away from the bottom end. The Sofitel on Silom gets a good write up but this is a 4-5 star hotel and not in the true luxury class.

In the Siam area I don't think there are anywidely accepted luxury hotels. The Intercontinental is a short walk away and is generally recognised as being an excellent 5* hotel. About 15 mins walkj away is the Four Seasons, widely accepted as one of the cities top luxury hotels.

I stayed at the Dusit Thani a few years back and whilst it does have loads of restaurants and facilities and then Landmark rooms and Thai Heritage suites were very nice, i think the general run of house rooms were lacking. A nice 4-5 star hotel, but not a world class luxury one IMO. It has been through a refurbishment though and it might be a different story now.