My preferred lock? None. I find them to be a pita... When I was kid we literally leaned our bikes against a tree in our front yards. No problem. Until there was. Times have changed.

What do I use? An Onshield Ulock and a master lock with a cable. For a quickie, coffee at McDonalds, I usually just use the masterlock and cable. At work the bike is in a room near my desk. At school, I use both locks. Last year my previous bike, locked with a master lock and cable, under the schools security cameras, was gone when I left class. Security showed me their video. After the sun set you couldn't see diddly. Yeh, I was pissed! The guard said to get one of those "U thingies, nobody steals those bikes." I went to the LBS and got one. So far so good.