A U-Lock is a widely used lock style that acts as an excellent deterrent to potential thieves. The bulky locky mechanism resists hammers, chisels, and similar tools. The horseshoe shape limits leveraging–as long as it fits your bike well–and limit the amount of space a thief can insert a crowbar. A cable lock is versatile and adaptable but will generally offer less theft deterrent than a U-Lock. Bolt cutters can cut through most cable locks, and on their own, maybe suitable for low-crime areas. Elsewhere it might be good to lock into a U-Lock as an additional form of security. A chain lock is another alternative that can hold up to high crime areas and is designed to resist hacksaws or chisels and the chain makes it tough to leverage. If you choose this option, you need to make sure you get a padlock that is just as tough. A negative of a chain lock is the weight of the chains while you’re carrying it from place to place.