I stayed at this hotel last week after staying at another nearby property. I don't hesitate in rating the Ritz-Carlton Istanbul a 5-star property, as it stands equal to other Ritz-Carlton hotels around the world in its amenities and service. The building itself looks slightly dated but still looks classy and charming. English proficiency of the staff was superb, and they were all very friendly and went out of their way to help.
The hotel did not provide requested bed type at check-in but offered an upgraded, larger room on the next day in addition to offering complimentary drinks at their outdoor lounge area, which has a beautiful view of the Bosphorus with a modern, trendy vibe (also has very tasty food).
One of the best features of the hotel is the hamam in the basement level of the hotel, which offers a small but luxurious Turkish bath area... perhaps not as opulent as Ayasofia but still very nice. We also enjoyed the food and service at the restaurant. The hotel's location is also great, as it is quite convenient to walk down to the tram station.
Breakfast could have been a bit better (especially the sausage), but other than this minor point, I simply cannot find anything to complain about.