That's a great and very practical question for anyone managing public spaces. In my experience, there's no single "most" vandal-proof lock, as determined vandals with time and tools can defeat almost anything. However, the goal is to use a lock that makes the effort so time-consuming, noisy, and difficult that it becomes a deterrent.
For a standard urban trash bin with a hasp, I'd recommend a heavy-duty, stainless steel padlock with a shrouded or protected shackle. The stainless steel resists weather and corrosion, which is crucial outdoors. The shrouded shackle—the part that loops through the hasp—is covered by a hardened steel collar. This makes it extremely hard to cut with bolt cutters, as the jaws can't get a clean bite. Brands like Abus or Master Lock offer good commercial-grade options in this style.
If you're dealing with persistent issues, consider a stainless steel discus lock or circular lock. These have no exposed shackle at all; the locking mechanism is entirely inside a rotating steel body. They are highly resistant to bolt cutters, leverage attacks, and even some drilling attempts.
For bins with integrated locking bars, a high-security cylinder lock is key. Look for one with a drill-resistant hard plate and anti-pick pins. The housing itself should be made of thick, hardened steel.
Finally, don't forget the hasp and mounting hardware. A lock is only as strong as what it's attached to. Use a heavy-duty, welded-on hasp or one secured with security bolts (like one-way screws or tamper-proof bolts) that can't be easily unscrewed.
The best approach is often layered. A robust lock on a securely mounted hasp, combined with a bin placed in a well-lit, visible area, provides the strongest deterrent. For the absolute highest security, some cities are even experimenting with smart locks and sensor systems, but for most applications, a quality shrouded-shackle padlock is the most cost-effective and reliable starting point.