Viajero Hostel Buenos Aires is new on the scene but has made a big impact. It's in San Telmo which is close to all the main sites.
There are always a long list of daily activities like tango lessons and city tours so this is a great hostel for anyone looking to get out and meet people. There are not one but two pools, one indoor and an outdoor pool open only in the summer.
There are shared dormitories and private rooms, all with air conditioning (crucial in summer!).