I've seen a lot of research into this to protect motorcycles, anything under 16mm is croppable but a 16mm chain weighs a hell of a lot. It's a trade off, how much weight do you want to carry? My bike is left in the centre of London every day so I have purchased the Kryptonite New Yorker 18mm D lock, it weighs 3Kg but is worth it as I can leave it places with fair confidence.Very interesting .... anyone know more?
Anything with a round type key is almost useless, it's common knowledge how to break into those in a few seconds.
Chains and D locks require different tools to break without one being the perfect solution, you need to know how to use each one for them to be of value.