You can easily get a cable or chain that would be impossible to cut with an average set of 18-30" ish bolt cutters and very difficult with a large set of bolt cutters. You can't really get something that cannot be cut with a cordless grinder with a zip disc, however that takes longer and more importantly makes noise MUCH longer than 1 snap with a bolt cutter.
If you want to defend against average bolt cutters, large cable, like 7/8" or larger, or high tensile chain. Any bolt cutters you can buy at the hardware store or Horrible Freight will not get through a 1/2" grade 100 chain. Then the lock becomes the weak link to get a lock that will go through chain like that and not be exposed to grinding or cutting.