Re: What are your favourite museums/art galleries?
I loved Galleria Borghese in Rome.
Peggy Guggenheim's collection in Venice was also great.
Others I really enjoyed: Gemaldegalerie and O2 in Berlin, Musee d'Orsay, Stadel in Frankfurt, Vatican Museum.
And my worst experiences (outside of Poland) were probably Bode Museum (way too much sculputures) and Grand Palais in Paris (they just had no idea how to handle the crowds)