For some reason I had the idea that getting money from an ATM in Poland would be the best way to PLN. Actually the exchange rate that I got in ATMs is very very bad compared to the Kantoors. Unfortunatly I brought a limited amount of money and I will have to use ATMs. Is there a best network/way to do that?
Many ATMs in Poland by default offer to exchange PLN into the main currency of your debit/credit card. Almost always the exchange rate they offer is pretty bad, but there is always an option not to use it - i.e. the ATM will charge PLN to your card issuer, who will use their exchange rate (and usually a fee on top of that) to debit your account. In my case (I am from the UK) it is much cheaper to use the latter option, but this depends on your card issuer (as mentioned in the comments).
Note that the same comment is true for some retailers.