Re: What are your favourite museums/art galleries?
My favorites are the Prado (Madrid) and the Metropolitan Museum of Art in NYC.
Next I love the Louvre, National Gallery in Washington, DC, and in London, the Victoria and Albert, the Getty, the Smithsonian, the Cloisters, the national museum in Athens, and the Vatican museum.
Basically any museum at all!
Definitely the Prado and the Metropolitan Museum are my favorites and I've spent more time in those places.