There are MULTIPLE posts about locks on this site. a quick search will give you more info than you probably want, including videos and quantitative tests.

All bicycle locking options can be defeated by a patient, determined, thief.

your desired level of protection will be dictated by (as megalowmatt said) where and for how long you will be locking your bike, and how much you are willing to spend. Many manufacturers offer an insurance policy against theft for the first year of ownership.

I use the masterlock streetcuffs ($50) and have had good luck but I rarely leave my bike unattended for more than 30 minutes at a time. Overall people on this site do not like/trust my choice.

to sum it all up, the "best" option according to BF appears to be the kryptonite new york city fahgettaboudit chain and lock. http://www.ems.com/product/index.jsp?productId=3660747