Onguard is another good u-lock brand that can often be found cheaper than Kryptonite. I scored an Onguard STD Brute for $35 on Amazon a few months back. I would shoot for something that is 14mm hardened steel or bigger. Anything less and they are susceptible to bolt cutters. A shackle that locks in two places is a good feature as it doubles the grinding time for the thief. I lock my u-locks to my rack and use a bungee cord to prevent them from rattling around.