Kryptonite - Kryptolok series 2 offers the best balance of security, practicality and price. It is a portable lock to secure your bike in the street every day. Kryptonite also gives you a lifetime warranty on products that have defects.
If you’re only looking for a lock to protect your bike at home or at work, then the best choice is a thick, high quality chain lock, like Kryptonite - Keeper 785 Integrated Chain. Because they’re flexible, they’re usually much less complicated to transport than U-locks.
Kryptonite offers a limited lifetime warranty on products that have defects.