face (that) would stop a clock
1. A face that is strikingly or shockingly unattractive. His physique was remarkable, but unfortunately he was cursed with a face that would stop a clock. She is extraordinarily bright, but her face could stop a clock.
2. Less commonly, a face that is strikingly or shockingly attractive. He walked down the street—tall, muscular, and with a face that could stop a clock—and everyone stared to admire his beauty. John's new girlfriend has a face that would stop a clock, so beautiful that it seems like time stands still when she enters a room.