Bike locks, as any other lock is only to keep honest people honest. I think most bikes are taken by opportunists so most locks will keep you safe from them. If someone wants your bike and they are in the business of taking bikes all you can do is slow them down. The best lock is the one that you will use every time you are away from your bike. I have a pad lock that will chip the blade on a 36-inch bolt cutter and have found that a frame straightening chain is resistant to bolt cutters as well. But neither will stop a good angle grinder. With the increase of 18 to 24 volt power tools it is pretty hard to get anything that will stop someone if they have time.
All you can do is try to find a place to lock your bike where people can see it and lock the wheels and frame whenever you can. For people that have to leave their bikes for a longer period of time getting a big U lock or ultra heavy chain with huge pad lock most often works the best. But remember this. If you get a lock that needs a cutting torch to get by it what will happen when someone puts super glue in your key slot? Don’t laugh, people do things like that.