Best way to pay for things when traveling in Argentina
In Argentina, our currency is Argentinian Pesos (ARS). The conversion rate at the moment is 1 USD = 70 ARS and 1 EUR = 80 ARS. However, inflation is a major problem in my country (over 50% in 2019!) so the exchange rate will definitely change a lot every year. If you take a look at the conversion rate between USD and ARS in the last couple of years you can see this trend:
Despite the inflation, cash pesos is still the best way to pay for things across the entire country as it's accepted by all businesses of any size from small local shops to fancy restaurants (except for hotels, it's always better to pay for hotels with credit card, more details below). Cash is even more prevalent when you go outside of Buenos Aires, so be sure to have enough cash on you to get around. Even in Buenos Aires, you can use only cash to pay for many things like taxis, and most shops in artsy areas like La Boca.
The best way to get cash in Argentina is to convert from USD or Euros at one of the many cambios in the cities (in Buenos Aires there are many cambios on Calle Florida). Avoid using banks or airport exchange shops because they use the official exchange rate, which is usually much, much worse than the unofficial exchange rate (called Blue Dollar Rate). For example, today the blue dollar rate is 1 USD = 117 ARS, but the official exchange rate is 1 USD =69.61 ARS, which is terribleCambios will give you pesos at the Blue Dollar rate. You can check the current Blue Dollar Rate on this website. Also, converting in larger amounts together will get you better rates, and if possible make sure the USD bank notes you bring are in good condition as some exchange shops may not accepted worn-out notes.
ATMs have become pretty popular for locals to get money in the last 10 years and pretty much all foreign bank cards are accepted, however they are extremely expensive to use than exchanging money or using credit cards, due to the fixed fees per transaction you will be charged by both the ATM and your own bank back home, as well as them using the official exchange rate. My advice is to avoid using them altogether, unless it's an emergency. You can read more about using ATMs in Argentina here
Credit cards are also widely accepted, especially in Buenos Aires and other major cities. Tourist businesses will almost always accept credit card, like hotels, restaurants, tours, transportation companies. Visa and MasterCard are the most widely-accepted. American Express and Diner's Club are also accepted by many businesses. My suggestion is to bring your credit card with you to pay for big expenses like hotels/hostels, flights, and tours. This way you can reduce how much cash you have to carry and and exchange. Credit cards are also cheaper to use than withdrawing cash from ATMs
Very important: it's always better to pay for your hotels in Argentina with your credit card. There's a 21% sales tax (VAT tax) that's added on top of everything in Argentina including hotel bills, but this is waived for guests that have a check in with a foreign passport AND paying with a foreign bank-issued credit card. So if you pay your credit card (assuming your card is not issued in Argentina), then your hotel bill will be 21% lower than otherwise. This is a great way to save a decent chunk of money here
One more tip I have for you to keep in mind is that if you are asked if you prefer to get charged in pesos or your home currency (like USD), always choose to get charged in pesos. If you choose to get charged in your home currency, the exchange rates can be up to 10% worse than otherwise. This is a pretty profitable thing for many Argentinian banks to make money from tourists
For safety reasons, avoid using your credit cards at sketchy places or any places that's not a major establishment. Card skimmers are a problem in Argentina and some tourists have gotten their information stolen. You should stick to using your credit card for only major purchases and upscale restaurants/shops
In addition to pesos, you can often pay cash US Dollars directly at many businesses (and you will see their prices listed in USD in addition to pesos). However, my advice is to not pay businesses in USD directly because the price they set in USD usually bake in a very bad conversion rate. Basically if something costs 100 pesos, if you pay in USD it may cost you the equivalent of 120 pesos, or worse. Obviously this varies by businesses but from what I've seen it's usually pretty bad