I have had very good experience of CISA padlocks (with Diamond Hard Chains) which resisted several persistent attempts by pickeys to break. The attacks included freezing with nitrogen, various hammering attacks, bolt croppers and a Stihl saw. Eventually they managed to break the gate and post to gain access to the field.... They lobbed the chain and lock (still locked) in the river!
It's what I have nowadays as they proved their worth. E.g.
http://www.cisapadlocks.com/cisa-285...lock-105-p.asp