That magical ATM is like the pot of gold at the end of the rainbow. We are all looking for it. If you find it please let me know. I have noticed that the Citybank ATM's seem to give the most pesos per withdrawal but then I looked at my bank statment and it seemed as if they were giving me a lower exchange rate than what was quoted for that day.
Citibank does give a much lower exchange rate than the regular bank rate. They admitted it when I asked them. When the dollar was trading at about 3.00 pesos per dollar, I was getting only 2.85 pesos from them. I stopped using them so I don't know what the difference is anymore.
Regarding getting a larger amount, I have noticed that the Banelco banks will only allow me to withdraw 500 pesos per day, whereas the Link banks permit withdrawals of up to 1000 pesos per day. But that may depend on the bank you use outside of Argentina and your agreement with them..