You can see the results of tests by a insurance organization in the UK at:

www.soldsecure.com/Leisure.htm

The locks rated "gold" by soldsecure are the same locks that scored the highest in tests by the editors of the UK magazine "Cycling Plus".

The test results at soldsecure are consistent with the "Star" ratings used on the Kryptonite website, and on the website for OnGuard locks. The locks that Kryptonite and OnGuard themselves rate as "best" also test as the "best" in independent testing.

The "best" bike lock that is easy to find in the USA and that is reasonably portable is the Kryptonite Fahgettaboudit U-lock. In the UK, the "Urban Paramount" received a rating equal to the best Kryptonite locks, and it may sell for a lower price.

The problems with the "best" locks is their weight. The highest scoring u-locks weigh five or six pounds. The highest scoring chain locks weigh eight to twelve pounds. So, for folks who carry their lock with them, it usually makes sense to use "medium" security locks that are more portable.

For example, the Kryptonite Evo Mini and the OnGuard Bulldog Mini TC each weigh about two pounds. They work very well against crooks who use manual cutting and prying tools. They won't last long against power tools, but in most communities, it is rare for crooks to use power tools.

No lock works if you use it wrong. Today I saw a bike with nice Campy wheels and quick releases. The u-lock was around the frame only. A crook could have taken both wheels in just a few seconds.

The correct way to use a u-lock is to place it around the rear wheel, just behind the seat tube. Do NOT put the lock around any part of the frame, as that allows a crook to use the frame as a lever to attempt to break the lock. Replace the quick release on the front wheel with a bolt, and use a cable lock, or a mini u-lock to secure the front wheel to the frame.