We've found that an American Lock, which is stainless steel and has a tubular key is the safest lock to have. Plus, as Ralph has indicated above, put it inside a highly tough steel box where you have to reach into it, especially if the reach is up and turned about 45 degrees, like a candy cane shape. Vandals have broken our heavy duty chain, but have never gotten by the American Lock.
Read the description in Google. The tubular key is very difficult to get past--In fact, probably not possible!