R12 would probably be the best refrigerant to use, because it has the lowest boiling point of any of them. I don't remember for certain what the boiling point is, but I think its pretty close to liquid nitrogen. Its also the one that was banned, so no need to worry about thieves getting that stuff. On the other hand, I don't care how good a lock is, a thief with a matabo will saw through it in a matter of seconds.