While using ATMs would generally result in the best rates, in Budapest, Hungary the best rates for € or $ are at the numerous small exchange offices in town. (The shiny ones, also at the airport are daylight robbery) If you intend to spend little cash (1-200 $) it's probably not worth the hassle, just go for the ATM at the airport and chose "I want to be charged in local currency" (!!) and confirm it. If you chose to be charged in your home currency, you'll lose 10-15% on the exchange rate.
The easiest way to see if they are a good deal or robbery is to compare the selling and buying rates. If the two are within a few-%, it's a good rate. If the spread is 10-20-30%, avoid them.
In Budapest you won't need much cash, mainly for taxis or smaller restaurants. Visa/MC/Amex are widely accepted.