The US.
At 8891 km, the border between Canada and the USA is the longest between any two countries, but a major section of that border (with Alaska) is not contiguous with the rest. The longest contiguous section of border between two countries in the world is between Russia and Kazakhstan, at 6846 km.
The only land border Canada has is with the United States.
Do you mean countries? If you do, the two countries that border the U.S. are Canada and Mexico.
Canada.
Canada
No. We have only ever and still do shared a border with the United States.
canada and mexico
Canada and Mexico.
Alaska and Hawaii do not share a border with any other US states. Alaska shares a border with the Yukon Territory of Canada.
Canada to the north and Mexico to the south border the United States.
Canada and Mexico
Canada and Mexico border the U.S.