What is the best bicycle lock on the market?
Some years ago, Bicycling magazine did a shootout on available locks and the Kryptonite "New York Chain" came out on top.  Unfortunately, it weighs more than many bikes and is not easy to carry either.
Fine if you can leave it where you lock your bike.

I'm with the police department at a major university, and we have a program through Kryptonite where we give the kids a (heavily subsidized) Kryptonite U-lock for a mere 20 bucks.
Over the 30 years I've been here, we've had only a handful of these locks defeated. Most all the bikes we have stolen (lots...) are locked with the sort of cable shown above.  The thief uses a rather small bolt cutter and removes it more rapidly than you could by using the combination.
The U-locks are easy to carry, not overly heavy, and offer a "good" level of security.

Anything can be defeated with the right tools, time, and opportunity.  Deny the thief these things and you have a better chance of hanging on to your bike.
Or....Buy a beater and save your nice bike for weekend rides.