I'd love to go back to the V&A. I enjoyed the National Portrait Gallery for some reason. I remember the Rothkos from the Tate Modern but otherwise it didn't leave an impression. Loved the Tower of London when I visited as a kid. But for me the best bit of London is just wandering around and running into places out of books - I have a thing for the 19th Century Novel, and grew up on very Anglocentric children's literature (Joan Aiken, Noel Streatfield, Arthur Ransome etc etc).