Image by Chance Hammock 500px Images
Peaking at 555ft (and 5in) and comprised of 36,000 blocks of stone, the Washington Monument is the tallest building in the district. It took so long to build that the original quarry ran out; note the delineation in color where the old and new marble meet about a third of the way up. Alas, the monument is closed until spring 2019 for repairs, so you'll have to wait until then for stellar views from the observation deck.
When it reopens, you'll need a ticket to get in. Same-day passes for a timed entrance are available at the kiosk by the monument. During peak season it's a good idea to reserve tickets in advance by phone (877-444-6777) or online (www.recreation.gov) for a small fee.