NPS has over 1,100 datasets: https://public-nps.opendata.arcgis.com
Washington, DC has over 900: http://opendata.dc.gov/
Parcels are probably the largest data that people publish on ArcGIS Open Data. LA's parcel dataset tops out at 2.4 million features.
Then of course there is opendata.arcgis.com which has all 68k+ datasets shared as open data within the ArcGIS platform.
I was looking for examples of a more curated space for an organization to present its authoritative data without irrelevant search results that often shows up in just arcgis.com. NPS and DC sites are very impressive.