Tate Modern
The Tate Modern is a big recommendation if you come anywhere close to London's South Bank. From slightly crazy Japanese pop art over impressionism to rock solid photography by Magnum veteran Bruce Davidson; You will certainly find something there that you like.And then, when you decide that you like galleries housed in post-industrial settings, you should probably get your bags packed right away for a tour of the Ruhrgebiet. They invented this kind of stuff.