If you live in a high risk area...buy a $50 bike.

If you don't live in a high risk area...several companies make high quality cable/chain locks, but remember, you get what you pay for.

Personally, I use a 1/4" chro-moly chain (anchor chain), and a Brinks Home Security lock. People laugh about the lock, but it's still going strong after 10 years of abuse and neglect. I have never oiled it, never cleaned it except to rinse it off, and I have put it through abuses that no padlock will ever go through. It has literally been shot, exposed to close range explosions, and left out in every weather condition this world has to offer from -40 degrees in Norway to 140 degrees in Iraq. As a Marine vet, I never dropped the habit of testing things until they break...this lock is the only thing I've ever owned that I couldn't manage to break on my own.