I have only and will only use one kind of lock and that is the Kryptonite Kryptolok Combo U:



And the reason is I hate carrying around keys that can get lost. I even have an electric combo lock for my house front door. Life is so much easier this way. A few months ago my best friend who now lives out of state had a unexpected business meeting here in town and wanted to spend some time with us afterwords. We were actually out of town so I gave him the combination to get in the house and the combination to the U-lock if he wanted to go for a bike ride. Saved him hotel and car rental!

If you live in a bad area then this lock probably wouldn't be good enough for you since it's not that thick. You would probably want to go with the OnGuard 5012C Bulldog DT Combination U-Lock & Cable