That's a great and very practical question for anyone dealing with pesky scavengers or outright theft in the city. Honestly, there's no single "most secure" lock that fits every bin, as determined thieves with time and tools can defeat almost anything. However, you can create a highly effective deterrent system that makes your trash can the least appealing target on the block.
The core of a robust setup isn't just the padlock itself; it's the combination of a heavy-duty locking *hasp* (the metal staple and loop) and a high-security *padlock*. Avoid flimsy, thin hasps. Instead, look for a heavy-duty, welded steel hasp that can be through-bolted to the bin and the wall or post. This prevents someone from simply prying it off. For the padlock, a shackleless disc lock or a solid body shrouded padlock is ideal. These designs protect the shackle (the U-shaped part) from being cut with bolt cutters or smashed open.
For communal dumpsters, integrated locking bars that secure the lid and are chained to a fixed object are often the municipal standard. In extreme cases, some businesses opt for tamper-proof one-way chutes that only allow trash in, not out.
Remember, the goal is "security through annoyance." Your lock doesn't have to be impenetrable; it just needs to make stealing your trash more trouble than it's worth. Pair this physical security with good practices: don't put valuables in the trash, and if possible, place bins in a well-lit area. This layered approach is your best bet for keeping your trash secure.