I've seen so manty paintings, I can't really remember which ones I like the best, I would suspect it is anything by Turner.
My favourite museum of all time was the Theatre Museum in Covent Garden, which I saw for the last time on my last trip to London back in November. It has since been closed. Despite an effort by the theatrical community of London, the Victoria and Albert Museum, under whose province it fell, felt that it could afford to keep it open and, of course, the government did nothing to help.
The Vickie and Al is also a great museum. And there is none greater than the British Museum. One of my favourites, however, is the British Library, where one can see items of great literary significance like the Magna Carta, Gutenberg Bibles, Scott's diary, and bits of Dickens and Carroll and Joyce and other great masters of literature.
I've always had a fondness for the Cincinnati Art Museum up in Eden Park.