Some watch brands do unique pieces, making sure to never duplicate a design or a certain combination a materials.
Jean Dunand only makes unique pieces, while companies like Patek Philippe, Piaget, Cartier, and others make occasional unique pieces, and these are truly special watches.
Designer Yvan Arpa's brand, ArtyA makes only uniques pieces, subjecting each case to thousands of volts of electricity, deforming the cases in a different way each time. Starting around $2,000 (and go up to $120,000 for complicated pieces), these are the most reasonably priced unique pieces.
ArtyA Son of a Gun piece unique, using real live bullets.