ATM can be the best, but using the direct debits to your account, rather than credit card. The efficiency of the operation will depend on how your bank handles exchange rates and transactions.

My every-day UK bank used loaded exchange rates, and levied a transaction charge each time I used an ATM. I switched to a bank which does not lift money from me in those ways. This means I can take money out of ATMs as and when I need it, without incurring financial penalties. I suggest you find out what the policy of your bank is.

You can use cc payments quite extensively in Brasil (small puchases, supermarkets etc), and if your cc company doesn't load exchange rates, this can be an effiicient way to go.

I always stick to billing in local currency. Merchants are eager to bill using Dynamic Exchange Rates (i.e. in your home currency) because they benefit from the process. You are very unlikely to benefit unless your home currency drops significantly in the day or so betwen transaction and posting. On the other hand, you may appreciate the certainty of knowing exactly how much you are being billed in home currency, and be OK paying a premium for that certainty: your call on that one.

Merchants are supposed to gain your approval before employing DER, but hotels can be pretty quick off the mark and often skip the prior approval bit.

For cash, exchange rates vary quite significantly among banks, Forex kiosks and agencies. Banks seem to be the worst. The airport counters can be sharks.

I have never used travellers' cheques in Brasil, or anywhere else.

Hope this helps.