That's a great and very practical question for anyone managing public spaces. From my experience and research, there isn't one single "most" vandal-proof lock, but a combination of design principles that make a mechanism highly resistant. The goal is to make tampering more trouble than it's worth.
The leading contender is often a high-quality, shrouded stainless steel padlock or hasp system. Look for locks with a hardened steel shackle, a shrouded or recessed body to prevent bolt-cutter access, and a disc-detainer or similar complex core that's resistant to picking and drilling. Stainless steel is crucial for weather resistance.
However, the lock itself is only part of the solution. The most effective systems integrate the locking mechanism directly into the bin's design. Think of internal steel bars or rods that latch into the bin's body, operated by a unique key or tool from the top. This hides the critical hardware inside, protecting it from direct attack. Another strong approach is using one-way, tamper-proof security screws or bolts to secure the bin's panels, requiring special tools for removal.
For high-risk areas, some cities use centralized locking systems where a single key operates multiple bins, streamlining collection but relying heavily on key control. The ultimate deterrent can be a simple, robust steel cable looped through bin handles and anchored to a fixed post, as cutting through a thick cable is noisy and time-consuming.
In summary, the best defense is a layered one: a premium, corrosion-resistant lock (like a shrouded stainless steel padlock) paired with a bin design that protects the latch points and uses tamper-proof fasteners. It's about increasing the effort, time, and noise required for a vandal to succeed, encouraging them to move on. Regular maintenance and prompt repair of any damage are also essential to this security strategy.