Choose a bicycle lock for the place and length of time the bike will be unattended. Sold Secure grades rise from Bronze through Silver and Gold to Diamond. Check the exact model in its current product directory; sharing a brand does not mean sharing a rating.
Fit matters too. A U-lock or chain should secure the frame to an immovable object, ideally capturing the rear wheel as well. Avoid unnecessary empty space inside the lock. Prioritise the frame rather than securing only an easily removable wheel.
Before buying, consider daily use: will the lock reach your usual rack, and can you carry it comfortably? A useful choice combines its security rating with a workable locking arrangement.