There are some great hotel options, in Buenos Aires, from traditional to contemporary.
I particularly liked the Sofitel at Arroyo 851. It is on a quiet street with many galleries and was a great place to be based to walk the city. It was the perfect size hotel with a lovely restaurant, bar, cafe and fitness center. The rooms were also excellent.
Hotel Alvear Palace: This is the grandest of the city’s old hotels. It is really beautiful, yet very formal. If you like a more traditional property then this is the place for you. Even if you don’t stay here, you can enjoy a drink in the bar or afternoon tea in The L´Orangerie restaurant.
Down the street from the Hotel Alvear Palace is the stunning Park Hyatt. Located in the renovated Palacio Duhau, a former palace, it has beautifully decorated rooms and restaurants.
On the more contemporary side is the The Faena Hotel, designed by Philippe Stark at Martha Salotti 445, the Home Hotel at Avenida Honduras 5608 in Palermo Hollywood and the very contemporary and sleek Hotel Madero in Puerto Madero.
Wallpaper* Magazine just recommended the Anselmo for great design. It is located in the San Telmo neighborhood.