The USGS elevation program was initially known as NED (National Elevation Dataset) when it replaced older USGS ASCII formats.  It also marked a shift from UTM projected data sets to arc second data, which was "seamless" and distributed from the USGS Seamless Server.  The current name is 3DEP, for 3D Elevation Program.

Bare earth DEM layers in 3DEP are nationally seamless and are distributed in geographic coordinates at 1/3, 1, and 2 arc-seconds.

This is arc second data of the highest quality available from USGS.  About 25% of the country has 1/9" data (about 3 m), but no more will be produced.  Alaska has 2" NED and a limited coverage with 1/9" data.

There is now a 1 m DTM, in IMG format (requires GDAL)  with a UTM projection in 10 km tiles which can be 250 mb in size.  It currently covers a limited area.  Merging data sets at the UTM zone boundaries is not as simple as with arc second data.

Download directions  from National Map Viewer.

One second NED.  Note that some water has elevations, and others is missing data.
One third second NED.
One ninth second NED.  Note half of a building in the center on the map.
1 m USGS UTM DEM.  This example is from the Pentagon.  In urban areas artefact dominate this product; the buildings and bridges/overpasses are mostly gone, but a number of the roadbeds are still prominent.  This is not the full resolution of the dataset.

NED references:

Last revision 10/13/2017