Which lock to choose to secure something outside.

The primary goal of a lock is to prevent things from being stolen. Sometimes you can't store something inside and you have to lock it in place outside. This can be a bicycle, generator, pump, etc.

A short list of the most commonly used locks and failure points:

Cable locks
Steel cable is made by twisting lots of steel wire in to a cable. This construction makes is vulnerable:
1: To cut a cable, you do not have to cut the full thickness of the cable. But it is possible to cut the individual wire separately. This allows cables to be defeated by fairly small pliers.
2:  To make cable you need a flexible kind of wire. To make a wire harder to cut you need a hard wire. Flexible and hard are contradictory. So cables are easy to cut with a bolt cutter.

Armored cable locks
Armored cable locks are cable locks with thick steel bushings protecting the cable. This makes it very hard for bolt cutters to cut it. However this design also has weaknesses:
1: There is a certain amount of play in the bushings, to allow the lock to be flexible. However this play is limited. If a long lever is used to twist the lock, the effective length of the bushings and cable will differ, causing the cable to be pulled out of the crimping and the end of the lock.
2: If twisting does not defeat the lock, twisting will expose small gaps between the bushing, allowing the cable to be saw through with a small hacksaw.

U-locks
U-locks are made made of relatively thick steel, making them hard to cut. The stiffness of the U-shape causes problems:
1: These locks do not stand well against torsion forces. If a bicycle is locked to a lantern pole, the entire bike can be used as a lever (by flipping the entire bike) to twist the lock open.

Segment locks
Segment locks are made of pieces of steel plate of tubing riveted together. This has the same problem as the U-locks:
1: These locks can be opened by twisting the lock, with a larger lever.

Chain locks
Chains combine stiffness and flexibility. This makes it hard to be cut by small pliers or to be twisted open. Since there are many kinds of chain locks, so here a list of things to look out for:
1. Chains can be made from regular steel or harden steel. Obviously harden steel is harder to cut.
2. A bolt cutter provide the most cutting power when it's almost closed. So it's best to make the beginning of the cut the hardest. A round shaped chain shackle has a relatively small surface area when a cut is started. A square shaped chain shackle forces a bolt cutter to cut a much bigger surface area a the beginning, making it harder to cut. A minimum thickness of 6 mm would be recommend.
3. If you need a small lock, get a lock with the closing pin covered by the last chain shackle when locked. This way the closing pin it self can not be cut directly.