Soldsecure.com has test results for bike locks, with the "gold" rated locks being the best.
Until recently, the typical cable lock did poorly in tests, lasting seconds, not minutes, against basic, cheap tools that would fit in a crook's coat pocket. However, Abus now has a steel-covered cable lock that earned a "gold" rating. It is expensive, and heavy, but might be useful for locking to poles that are too thick to use a u-lock.
It is a rare city that does not have poles that are a good fit for the Kryptonite Fahettaboudit u-lock, so for most folks, a "gold" rated u-lock is still the least heavy, most secure way to lock a bike.
I sometimes see $2,000 bikes locked up on a public street with no owner in sight. There is no lock in the world that can stop a determined, skilled, experienced professional thief from taking a bike he really wants. If you must leave a bike out of your view for more than five or ten minutes, you need a bike that is worth less than $300 or $400, or an expensive bike that is so old, beaten, battered, and dented that it LOOKS as if it is worth very little.
For a "cheap" old bike, left alone for a few minutes here and there, I like to use the Evolution Mini u-lock or OnGuard Pitbull Mini TC around the rear wheel, and a second mini u-lock or beefy cable lock to attach the front wheel to the frame. Although a skilled crook can defeat both of those locks quickly, "top" crooks are gonna invest their time in bikes that can be pawned quickly for "top dollar"...an old "beater" bike won't earn him $10 at a pawn shop.