I would take a look at the Fasano group. It's one of my favorite hotel brands. Probably the best designed hotels in the world. Fasano Rio de Janeiro is a personal favorite of mine. The service is great after South American standards and this is the hotel I would travel back to South America for. (+ Rio is a great city)

If you are open for Ecuador and Peru I have other recommendations.

Hacienda Rumiloma is a very special unknown hotel in Quito with only 7 rooms and great food too. Could easily be a Relais&Chateaux.

After reading FlyerTalk I was very excited about staying in Monasterio in Peru, but I consider Miraflores Park Hotel in Lima a better hotel in many areas. If you're looking for great value Tambo del Inka is a great choice on the way to Machu Picchu. Their occupancy has been less than 20% for a long time and that's a shame. It's a beautiful property, well managed and with only one problem. The English level of the staff, but you'll find that in my properties in South America. Tambo del Inka costs around USD 300 pr. night, but even better value if you book with SPG points.

Alvear Palce if you can like bigger hotels. I have tried all kind of hotels and I don't like staying in the big hotels anymore so it will depend a lot on your personal preference.

Henrik