Only two things work as a lock on a bike and that is your hand or your Butt. Keep one of them on the bike and it won't get borrowed. But there are several good locks about and the one I use is a Kryptonite. I use that to lock the frame to an immovable object and use a hawser to through the D Lock- wheels and saddle rails.

But even the best lock will only delay a thief if he wants your bike. The better ones delay for longer.