Perhaps if we knew why you ride a bike, that might help. 99.99999999% of my riding is only for recreation and fitness. So I never have a reason to be far from being in sight of my bike. If you are riding your bike to shop, go to school or doing other things with it that might have it outside and out of sight for a period of time, then might change some of the responses you get to your inquiry.
If you do have to leave your bike unattended while you shop or go to school, then I'd think any lock that is big enough to be very obvious will deter the casual thief just wanting a joy ride for the moment. A six foot cable with red sheeting on it and a padlock is what I used when I was in elementary and high school. Looped though both wheels and frame and fastened to the bike rack it wasn't going anywhere as easy as the unlocked bikes next to it. I never heard of those unlocked bikes being stolen though.