My favourite (at the moment) is the Tate. I haven't been there personally but I have 'toured' using their website. I'm a fan of J.M Turner and the Tate has one of the largest online collections of his work. You can even 'flip' through his notebooks. Here's a link: www.tate.org.uk/servlet/BrowseGroup?cgroup­id=­999­999­998­

Also, there's a Museum club at: link

My Favorite Art Museum Is El Prado (Madrid) Spain

Musee Mamottan Monet and Hermitage museum :-)