We recently had an issue where one of our U-locks stopped working. We asked for feedback on our Facebook page.
Here's what other cyclists recommended:
Abus Granit Plus 640
Jason and Andrew recommended the Abus U-locks. The Granit Plus 640 was specifically recommended.
The TiGr Mini is super-lightweight and made of titanium and comes recommended by Scott.
Kryptonite New York
The Kryptonite is a beefy, high-security solution that was recommended by Warren.
Another lock that we're considering is the Litelok Gold. Lightweight and easy to use.