Top of the line is Abus. What are you locking to? Bike tethers set in the concrete that can be reached by a U-lock? Abus & kryptonite have substantial models. Both have folding bar locks for achieving the same security points. Both devices are popular enough that in high theft areas thieves frequently carry battery grinders to cut through them. Short tether fixes the device to hold still when being ground. Some thieves cut the mild steel fixture rather than the hardened steel lock.
My city provides racks to lock the front wheel, allowing the thief to carry off everything expensive in his van. I lock to electric poles, gas meters, and live electrical conduits. While square chains like the pagus from westechrigging.com are hard to cut, stainless steel cables as I use are lighter. 3/8" x 6' sling has kept my bike safe for 3 years. 1/2" stainless sling may be more appropriate for more theft prone areas. Abus granite is the premium lock for use with slings. A reviewer has noted that granite takes at least 2 shots of a gun to make the cylinder fall out. Slings are floppy up in the air and take 2 hands to hold still while the 3rd holds the grinder to cut effectively. My 3/8' sling rolls up to 14" diameter to fit in my pannier bag. chains fold up much tighter but weigh twice to 3 times as much for the same length.
For severe theft areas motorcycle shops sell special alloy chain that weighs a lot and takes a long time to cut through.
Note cables specifically sold for bicycles are garbage. I can cut the lumous or master cable with a hand diamalloy cutter. Not even a bolt cutter. the ones with built in combo lock, those can be smashed with a hammer or a rock.
An impenetrably strong lock which conveniently attaches right to your bike. Solid, hardened-steel with drill protected rivets make bolt-cutters a non-issue Features Technical Contains Foldylock Classic review
I found better prices from UK. What indianajo said about weight of chains hits home now that I have used my chains and locks for well over a year. Two heavy chains with heavy locks and one u-lock from Abus in the paniers, sometimes with a cable as well, adds up to a lot of weight. Locking different parts of bike to different items ensures more work to steal the bike. Also have an alarm.
Oddly, after a year, doing the procedures to lock up faithfully has become somewhat enjoyable. Maybe it's because I know my bike is the least attractive one to steal anywhere I have locked the bike up, but there is something psychologically pleasant, theoretically, about doing healthy habits and now I can agree with that because I notice it as I take the time to lock up and unlock.
I would opt for strong slings like indianajo does, if I were to purchase again.
Also, I will never lock my bike up for more than 10 minutes in some areas of Toronto because every bike on the block, if not stolen, is stripped or smashed. Think Kensington Market/University of Toronto area.
Our city is now "woke" trash.
That 2.2 m kryptonite cable @ $17 may be as cuttable as the lumous I bought for $18. Tiny carbon steel wire that can be cut 3 or 4 at a time by a hand cutter. I gave it to my former yard man, who never locks his bike up whatever he has got. A $3 lock too. The 3/8" x 6' SS sling I bought from mcmaster costs $45 plus UPS charge. The 1/2" one is $63. Just cutting one strand of those would be a pi**.
I can't help thinking a lock with an alarm will suit me fine. Every situation is different though. If you have a super uncuttable lock it can still be cut given the right tools so it really comes down to where the bike is parked. A cheap lock with an alarm works for me to just go get some groceries. Locking it up in an alley would be far differant.