If the credit/debit card does not charge a foreign currency conversion fee and you pay the credit card off before it accrues interest then a credit/debit card will be the best way to reduce currency conversion expenses. Traveler credit cards, such as those that are with a frequent flyer program, typically have no foreign currency conversion fee. Additionally, most credit cards conversation rate seems to be better then what individuals get, probably because the buy foreign currency in bulk.
Second least expensive
Withdraw the amount of money you need at an in country ATM. There will be a normal ATM withdrawal fee, but it's a fixed amount. However, if you withdraw too little you will have to withdraw again, incurring the withdrawal fee. If you withdraw too much you'll have excess currency that you will have to sell, probably at a less than favorable exchange rate.
These are both generalizations. It's possible that you could find a favorable rate at a currency exchanger or the exchange rate could fluctuate enough between when you convert and when you get to the country your visiting to make it worth you while. I would not expect either situation though.