A sturdy u-lock is one of the widest used bike lock styles for a good reason. The bulky locking mechanism resists opening with brute force. U-locks come in a variety of sizes - your goal is to size the lock so that it goes around the things you’re locking with as little space left as possible. Small to medium models lock one wheel and your frame to a fixed object. Large models lock both wheels and your frame to a fixed object.