It depends what you find convenient and what you are prepared to pay for convenience.
Using a debit card in an ATM to get cash and using your credit card for larger purchases will cost you least in charges and exchange commission, and is mostly convenient.
Travellers cheques (even 'commission-free') will cost a little more, but providing they are in US dollars will cost you nothing to spend or encash there. But any you have left over you'll get charged for again when you cash them in at home. They can be replaced if lost/stolen.
If you don't want the bother of finding an ATM at the airport as soon as you arrive, take a bit of US cash with you.
Don't take sterling, cash or travellers cheques, except to use when you get home - they will be next to useless