What's the hardest lock to break into?

What is the hardest lock to break into is subjective but security professionals generally agree that high security mechanical locks like Medeco M4, Mul-T-Lock MT5 Plus, and Abloy Protec2 are among hardest to pick, drill, and bump due to patented key control, hardened steel inserts, and sidebar mechanisms that resist traditional picking. For smart locks, hardest to break into would be Grade 1 commercial rated locks with encrypted communication and no known vulnerabilities, such as Schlage Encode Plus or Yale Assure Lock 2 with proper installation. However, no lock is unbreakable, all can be defeated with enough time, tools, and skill, so hardest lock concept includes resistance to picking, drilling, bumping, and brute force. The absolute hardest would be a bank vault lock or safe lock with multiple layers. For residential doors, a high security deadbolt like Medeco combined with reinforced strike plate, 3 inch screws into studs, and secondary lock provides highest resistance. Abloy Protec2 disc detainer lock is considered virtually unpickable and is used in high security applications. Ultimately, layered security with good door, frame, and alarm matters more than single lock.