Chains are used to hold things together. So when you break one apart by sheer strength in makes for a great feat of strength.
There are a variety of ways to do chain breaking feats. Common ones include chest expansion as pictured here by oldtime strongman Pierre Gasnier. A similar one is to break the chain over the flexed upper arm.
Others include pulling the chain apart in the hands or in some sort of a deadlift movement with the chain attached to the ground as you’ll see in the video below. Of course there are many other ways this feat could be performed with a little imagination.
These feats are usually done with single jack chain. A good starting point would be #10 jack chain. And for the truly superhuman you could probably work down to #6.