Robust, strong, safe: Chain locks are the nightmare of any thief
Their extremely robust construction makes chain locks a mighty weapon against theft. They can only be picked with heavy equipment, much time, and great effort. Chain locks make thieves throw in the towel before even trying to touch your bike. The best way to secure your bike is to lock it to a tree, a lamppost, a railing, or a bike stand anchored in the ground. The chain links are made of specially hardened steel, the lock cylinders and lock cases are quite complex and designed to withstand strong external forces. By now, it should have become clear that the safety level of high-quality bike locks come with a price tag, no matter if you prefer chain locks, cable locks, U locks, or folding locks. But the price is justified if you consider the costs and the trouble when you have to buy a new bike.
It depends on your personal preferences whether you should choose keys or a combination lock.
Manufacturers like Master Lock, Contec, and Trelock have the perfect chain lock for every taste. They come in different lengths and also with different locking systems. Reversible keys have proven their worth. They make it easier to close and open your lock when there is only little space. Combination locks have the advantage that you can't lose the key and don't have to add another one to your key ring.
Further advantages of chain locks
Chain locks are flexible and, depending on their length, easy to use. Simple designs are suitable for basic theft protection, i.e. if someone tries to drag the bike away. If you're using your bike in urban areas only, a heavy lock would be recommendable, also because of its discouraging appearance.