Best option to get Pesos with credit card in Mexico?

Bank ATM rates change automatically to keep up with the exchange rates. They are never constant. You may make two withdrawals in the same day only to find that the exchange rate has moved up or down.

S&P dropped Mexico's rating from stable to negative and I was expecting a sharp increase in the price of the dollar. It went the opposite way.

Most Mexicans use a casa de cambio for exchanges under $10,000. The prices are much better than banks.