Palace of Westminster
Houses of Parliament along the Thames, which also Big Ben is a part of, is one of most well-known destinations in London. The original palace was built on the 11th century, and the current one was completed 1876.
The building is familiar from movies, postcards and posters. If you want to get a better overall view of the whole building, it is better to go to the other side of the river, where you can see the more familiar side. You can also get pictures from the Westminster Bridge next to Big Ben, although Big Ben itself is currently under renovation.
The nearest underground station is Westminster on the other side of the road (Circle, District, Jubilee). There is also a pier at Westminster which is accessible via the Thames Clipper.