In my opinion the best is Wroclaw Zoo: the biggest and most various collection, some rarities among mammals, birds and reptiles, good breeding results, a lot of new pavillons and enclosures opened in last years, very good perpectives for nearer and further future.
Second place for Poznan New Zoo (but from april to september only, in colder months you couldn't see a lot of animals - there are only two houses with access for visitors). Good collection of mammals (ungulates, small carnivores, rodents) and birds (pheasants, owls, birds of prey) - among them some rarities, big and nice South America enclosure, very good night house and new, big elephant house.
Third place for Warszawa Zoo and Opole Zoo. Warszawa have big collection and some good or new pavillons: free-flight aviary (the only one in Poland), elephant house, ape house, hippo house with underwater viewing.
Opole have good collection of south american monkeys and lemurs, new pool for sea lions with underwater viewing, fine savannah enclosure, nice insect house. Zoo looks very pretty, really nice place.
Next are Krakow Zoo, Gdansk Zoo and Plock Zoo. Krakow Zoo is nice located in forest on outskirts of the city. Good collection of ungulates, small carnivores and waterbirds, small night house, new enclosures for big cats.
Plock Zoo have various collection: south american monkeys, reptiles, fish.
Gdansk Zoo have very nice location, big possibilities and good plans for future.
Next are Lodz Zoo and Chorzow Zoo (also knows as Katowice Zoo - it lies between these cities). Both zoos have some interesting species (Chorzów has biggest collection of antelopes in Poland) but have poor financial support from local authorities - almost all zoos in Poland are municipal properties.
Other zoos: Torun, Bydgoszcz, Zamosc, Swierkocin, Borysew are rather local attractions.