The best melting cheese for soup is generally cheese with a smooth and creamy texture that melts easily. Some popular choices include cheddar, Gouda, Swiss, and American cheese. Cheddar cheese adds a rich and sharp flavor, while Gouda provides a milder, smoky taste. Swiss cheese offers a nutty and slightly sweet flavor, and American cheese is known for its excellent melting properties, creating a creamy and smooth consistency in the soup.