Breguet Grande Complication Marie-Antoinette: $30 million

This one-of-a-kind watch was designed by Breguet himself! It has 823 parts (more than a smartphone). The original watch was never completed as Breguet died before he could finish it and the partial watch was never found. Swatch has since reproduced it from the original plans. They have received offers upwards of $30 million, but they (foolishly?) declined to sell the watch.