Zürich clearly. But seriously no idea. I just heard Zürich was designed more or less based on the wishes Google employees had in the already existing offices.
Cracow office is pretty cool. It's located in a (not very large) historic building, so there's no open space. Also, the location is amazing - it's right by historic/touristy Market Square, with hundreds (thousands?) of cafes/bars/restaurants within a short walk.
As non-Googler I've been to the country HQ's in Dublin, London, Oslo and Sydney.
I think Dublin is the best. Oslo is the most basic but is understandably small.