Pragmasis 19mm chain and squire custom lock to fit with Pragmasis anchor. One of only two chains that couldn’t be bolt cropped by the guy in the video with 42″ lewins.
The lock is so tight a fit you can’t get leverage in it.
The key to a really good lock is the hardening. Most locks are too hard, as has been said freeze can then work easily and smashing, if hardened correctly they just king of dent with that type of abuse.
Diamond disc cutters still work but take 15 to 20mins.
I had big motor bike lock before with big motorbike padlock. They cropped it easily, too hard, not thick enough.
Good chains cost a lot of money ~ £250 but are the best deterrent, but that’s all they are, unfortunately. Just do the best you can and be well insured plus don’t get too attached and keep records of all parts/bikes in detail with pictures. Makes life much easier later if you have to claim.