Reese Towpower 7030200 Professional 5/8" Stainless Steel DogBone Style Receiver Lock
Reese Towpower Professional Locks are the most theft resistant locks in the towing industry.  Their superior strength is derived from a patented locking design that does not require milling a grove into the pin that creates a weak point.  Weak points allow a thief to break the pin more easily and escape with your trailer or hitch accessory.