The first thing you will look for is where to stay. As an expert tour guide I have been picking up people at most of the famous hotels. I work private or small group tours so I have been in all the 4 and 5 stars hotels and hotels boutique. I highly recommend to stay at those areas: Palermo; Recoleta; Puerto Madero; Retiro; Downtown; San Telmo.
Of course all depend on your budget and your age and style. Just in 2 lines:
Palermo: is far from the touristic points but has a lovely enviroment full of local designers, street art and nigthlife
Recoleta: is a very elegant place and close to everything.
Puerto Madero: very nice place also is close to the port and downtown. Lot of restaurants around
Retiro: Same as Recoleta. Elegant. May be not so close to go for dinner at night by walking as it is little dark sometimes but is a very good option!
San Telmo. Is not my favourite . Why? is a mixture of bohemian and historical streets but at night or early sundays can be little dangerous for my point of view
Downtown . As all big cities is full of thins during the day. Close to the leather shops, shopping mall, and crowded. But at night that is totally different.
Hope that helps. Hire me! http://guidesfinder.com/lizandrea-r/