Monzo is your best option, there’s no fees (even hidden fees, like the one’s your “no commission” services charge) for conversion of currencies or using ATMs. Foreign ATMs may charge you for withdrawing money, of course but that’s not a Monzo charge.
You check the MasterCard exchange rate here.
Take a look at this blog post for more tips
https://monzo.com/blog/2016/07/06/how-to-travel-with-mondo/
You could also try creating a ‘Monzo in country name’ topic, to ask for advice about using Monzo in the countries you’ll visit, from other Monzo users. Let us know how you got on when you get back 
The existing travel topics are listed here -