The only place you want to use there is Banco de la Nacion which gives you the official exchange rate. I guess they open at normal business hours, so around 8-9AM? I might be wrong though.
Stay well clear of all the other exchange places! Recently I was at the airport and checked the exchange rate at the other places. The official rate was 3.85 to a dollar, they were offering 3.33 to a dollar! And people were still queuing up to use their service. (The only thing they had to do was looking to the left and walk another 50 ft. BdlN was advertising the rate on a display!