I'm arriving to Wroclaw airport (Ryanair) next April, where should I exchange most of my money? In the airport, in the city or in another city?
There are plenty of ATMs and Exchanges all around the city, especially in city center.
At ATM, you will get a rate controlled by bank and Exchanges have their own rates. They may be a bit less interesting at airport (I suppose), I would suggest exchanging money somewhere at market place or Galeria Dominikanska.
However, ATM seems more convenient (at least to me).
Not sure of the arrangements in Poland but in Ukraine the best way to get local currency would be to simply withdraw it from an ATM. All the exchange places would give you worse rate then you would get for the electronic money transfer from bank to bank. And from my exprience airports are the worst.
Generally, almost wherever I go, I find it's easiest to get money out of the ATM. (Aside from Uzbekistan, where ATM's were card-specific). Ideally if you can before you go, arrange for a debit card or similar with zero fees for withdrawing in other countries - your bank may be able to offer you one, or you can order one online. Some really good tips on MoneySavingExpert.
I'd advise to visit Kantor Cent at the city center, prices very close to FOREX market, much better than bank's rates.