What is the best ATM card to use in Spain, lowest or no transactions fees.
Start with credit unions, capital one debit card and check other banks in your area. The best card for Spain is the same as France, Germany, Italy, etc.
I use USAA. They charge a one percent flat fee which I think is fair. It's not too hard to find credit unions/small banks thta offer that rate. There are some free options, although they come with deposit, brokerage account, or other requirements. If the requirements work for you, take advantage of them. The top end, as far as I know, is Wells Fargo and some other big banks. They charge a $5 fee PLUS 3 percent of the transaction. Having, at least, a travel account at a more reasonable financial institution can easily save $100 or more during a trip.
As Frank said, start by shopping your local credit unions. Mine charges absolutely nothing, and I just have a regular checking and savings account, no special account or minimum balance required.