No Canadian City Borders New Jersey—They're in Different Countries

The question contains a geographical misunderstanding: New Jersey is a U.S. state; Canada is a separate country. They do not share a border.

• New Jersey borders: New York (north/east), Delaware (southwest), Pennsylvania (west), Atlantic Ocean (southeast).
• Canada borders the U.S. along the 49th parallel (west) and Great Lakes/St. Lawrence (east)—hundreds of miles from New Jersey.

The closest Canadian cities to New Jersey are across Lake Ontario/Erie:

• Toronto, ON – ~350 mi (560 km) from Newark.
• Niagara Falls, ON – ~400 mi (640 km).
• Kingston, ON – ~300 mi (480 km).

Travel requires crossing an international border (passport/visa). There is no Canadian city "closest to New Jersey" in a bordering sense.