Spotting at Washington Reagan National Airport
A recent report from spotters visiting Washington Reagan National Airport states that Gravelly Point Park, just off George Washington Memorial Parkway, is an excellent spot.
This is particularly the case if aircraft are landing from the north, as they will pass very close to you.
If aircraft are landing from the south, you can still read off most movements from this location, although photography may be unfruitful.
This also seems to be a location that is tolerated by the local authorities, as the report states the spotters had no problems being there.