I've got this one.
http://www.abus-bordo.com/html_en/bikelock_bordo_lite.html
It's heavy, but very strong. Unlike U-locks, it is easy to carry on the bike.
Update : video showing operation
http://www.youtube.com/watch?v=57zc9kregqs&feature=related
Pretty simple : unfold like an accordion, wrap around frame, front wheel, and some "urban furniture", lock. It only takes a few seconds.
The short version is a bit short. The long version is ... longer but heavier.
The weak point of U-locks is the U-lock holder which is always made of crap and will break at the worst possible time, like hitting a pothole while going fast. That happened to me. It's annoying.
This one has a neat holder, very easy to use, doesn't fall off the bike.
Safety :
Doesn't open with a pencil.
Breaking a good U-lock requires a scissor jack (easy to find, you probably got one in your car) or some lock-picking, or an angle grinder.
This one is immune to the scissor jack, and very hard to saw, so you'd need an angle grinder (VERY noisy) or a specially machined key (I can't believe this age-old lockpicking technique still works)...
If you want more info and tests of many locks, check this french site :
http://bicycode.org/html/pg_htm/antivols.php
http://bicycode.org/html/telechd/antivols-testsFUBicy.pdf
The test results are in the PDF (updated yearly).
- 1 star : resists discrete attempts using tools not larger than 30cm, corresponds to thieves operating in public
- 2 stars : resists much less discrete attempts using tools not larger than 75 cm, and a much longer time "allowed" for the attemps. Corresponds to thieves operating in at night with noone to see them.
The test report has weights and security level for many locks, I hope you find that useful.
Best security
Nope, but best security means 2 locks which need 2 different tools (like a frame lock and a U or bordo)
Easiest usability when securing your bike
Yes
Easiest method of transport
Yes definitely
Lightest without other compromises
No, it is heavy as hell (like a U) but does that matter a lot for the daily rides to work or groceries ?