I've looked into this a while ago and my conclusion was that a good u-lock (Abus, Kryptonite) should be very hard to pick to someone who isn't the lockpicking l

I've looked into this a while ago and my conclusion was that a good u-lock (Abus, Kryptonite) should be very hard to pick to someone who isn't the lockpicking lawyer , is hard to brute force and is portable. A thick chain might be better but is too heavy to take on a bicycle. Anything else is too easily bruteforced. Always lock your bike to something or they can just walk away with it. If you're really paranoid you can take two u-locks from different brands and leave a chain at your work and home. At that point the weak link will probably be the thing you're locking your bike to.