What is the best form of money to use when traveling overseas?
The local currency is. many small merchants , in fact , most will accept nothing else. If a larger place will accept your home country currency, they will NOT give you a good exchange rate take your ATM card and use it to get local currency as needed.
1. A good credit card
purchase everything that you can using the card (food, lodging, etc.)
use it to get a small amount of cash at the cash machine at either the airport or train station upon arrival. Use cash only when you cannot use your credit card. (on street vendors, small shops, public transportation (if you don't buy a pass)
believe it or not, this is the cheapest way. The amount you pay the credit card company for transactions is usually much less than what you pay to exchange money.