Love GOV.UK design guidelines. Working for a government department at the moment as a contractor building some internal frontends and the design system is used internally as well. Really helps keep things consistent across all areas of government.
UK
– 10 Downing Street https://www.gov.uk/government/organisations/prime-ministers-...
– HM Treasury https://www.gov.uk/government/organisations/hm-treasury
– Department for Education https://www.gov.uk/government/organisations/department-for-e...
US
– White House https://www.whitehouse.gov/
– Department of the Treasury https://home.treasury.gov/
– Department of Education https://www.ed.gov/