Kryptonite New York Fahgettaboudit Mini U-Lock is quite possible the strongest Ulock for bicycles in the World! This extreme bicycle U-lock handles the urban areas, college campuses and other high theft danger areas.
I actually feel that my bike is safe and can leave it anywhere without worrying if it's still going to be there when I get back. The lock is very heavy given how small it is. It's pretty easy to carry around except for the weight, but then again, a light lock would be easy to break. If you need a high grade lock and don't mind the weight, this is an excellent lock.