Please tell me about the exchange kiosks that offer the best rates for US$ cash. Thanks, I'm leaving for BA tomorrow night.

The places cannot be named as this exchange is normal but illegal. Also advertising goes against the rules of this forum.
Without naming any places, we can help a foreigner who does not know how it works and might get in trouble if he deals with the wrong people.
The "official" exchange bureaus do not give you the best rate. But if you walk along the main tourist pedestrian mall (Florida) you will hear lots of people saying "cambio" (exchange). If you approach one ask him how much, they will ask you how much you want to change and what denomination you carry (20s, 50s, 100s), then will give you a quote. If you agree they will lead you to the inside of one of the many magazine kiosks in the street, or the backside of an "official" exchange bureau, and you exchange your money there. That works fine in the vast majority of cases. But if they try to take you to a private building, turn around and get lost. Those are the (few) cases when people get robbed.
Follow up: Calle Florida seems to have at least 30 "agents of change" for every block. The rates are pretty consistent. I got 12.55 to the dollar as of 2 days ago.
I'll just say that you should avoid receiving torn bills in your exchange as not everyone wants to accept them.
Frank, I'm glad it worked for you. I forgot to mention the torn bills -indeed, many unofficial exchangers only accept crisp, fresh off the bank notes.
