What Canadian province borders the most US states?

With actual land borders, it is Quebec with four - Maine, New Hampshire, Vermont and New York.

If you count an expanse of water between a province and a state, the winner is Ontario with six - New York, Pennsylvania, Ohio, Michigan, Wisconsin and Minnesota. One might further argue to include Indiana and Illinois at the southern end of Lake Michigan, but the way the "border" cuts across Lake Huron and Lake Superior, Michigan pretty well blocks them.

In terms of land borders, it's actually a tie between British Columbia and Quebec, which each border four U.S. states. British Columbia is bordered by Washington, Idaho, and Montana to the south, and Alaska to the northwest.

http://www.geographicguide.com/north-america-map.htm