That's a great question, and a surprisingly common problem! To prevent someone from walking off with your entire outdoor trash or recycling bin, you need a lock that deters both casual opportunists and more determined thieves. The most secure type generally involves a heavy-duty, weather-resistant padlock used with a sturdy locking hasp or a bin specifically designed with an integrated locking system.
For maximum security, a hardened steel shackle padlock (like a disc or shrouded padlock) is top-tier. Pair it with a heavy-duty, rust-proof hasp that you can securely bolt or rivet through both the bin's lid and its body. This setup makes it extremely difficult to cut or pry open. For a cleaner look and good security, consider a trash bin model with a built-in locking lid that uses a key or combination. These integrated locks are convenient and robust.
For a smart solution, a heavy-duty bike lock (a U-lock or thick chain lock) threaded through the bin's handle and around a fixed post or railing is very effective. It physically anchors the bin, preventing the entire can from being taken. Whichever you choose, ensure the lock is rated for outdoor use to withstand the elements. Ultimately, the "most secure" lock is one that is consistently used and combines physical strength with a method that anchors the bin to a permanent structure.