Oxford Monster OF803 Chain and Padlock
I have been securing my bikes with one of these for a number of years. It's very heavy and comes in a bag that is supposed the fit on the rear seat or rack. Over the years I have tended to have it in the top box (where fitted) or latterly in one of the Rocket's panniers. It also takes up a lot of space. I bought it as it got good reviews and has passed a couple of theft tests. Nothing is 100% safe from the scum that steal bikes but the time it takes to break or cut this one means they are exposed to being seen for a long time. I paid about £80 and I see that they haven't gone up much in the intervening years. Amazon selling them on Prime for just under £85. A big chain is great when used alongside another lock such as the disc lock or a U-Lock.