Consider the Alvear Palace if you want an opulent hotel experience. (Allegedly, there's an upscale adult entertainment venue across the street above the Cartier.)

I also can't say enough positive things about the Dazzler Flats Quartier on Basavilbaso Street (in Retiro, but a stone's throw from Recoleta). The rooms and furnishings are brand new, although it is not a full service hotel. But for $55 a night, it was one of the best travel purchases I've ever made.