Most of the brands make U-locks in both full size and mini-size. If you're always locking to regular bike racks, the mini-size should be big enough and it will be substantially smaller and lighter-weight than the full-size. Kryptonite, Abus, OnGuard, etc. all make fine locks. I would stay away from their cheapest U-locks that are made from cheaper steel that is easier to cut. Wirecutter.com (part of the New York Times) recently did a review of bike locks which is a good place to start.