Believe it or not it depends on two things, how expensive of a bike are you locking up? and is the area you locking it up have a high crime rate?
If the bike is a $500 bike why lock it up with a $200 lock? if the bike is a $5,000 bike why lock it up with a $20 lock? Regardless if $500 or $5,000 bike I would use 2 locks of different materials or styles like a U lock and chain etc. The best lock to use to protect a $5,000 bike is to buy a $300 or so bike (used or new) and lock it up with $45 in locks and save the good bike for rides where you won't be locking it up.
Here's a little web site of the best locks that were tested, see: bicycling.comsmash lab bike locks brokenOpen ↗
This is the best lock for under $50 for low priced bikes but I would add a second pad lock or Abus Discus lock for the cable instead of attaching it to the U lock because once that U lock is broken the cable is useless, anyway see: thesweethome.combest bike lockOpen ↗
Here is how to lock your bike; see: kryptonitelock.combicycle lock up.htmlOpen ↗
But I feel it necessary to warn you about something, and that's the bogus lock warranties that supposedly will pay X amount of dollars if you bike is stolen while using their locks. Bogus? say what you scream? Yes bogus because of all the loops you must go through in order to get the payout. Loops like the ones Krypto has: You must send in your original receipt and original UPC code cut from the box and send it in within 10 days of purchase. No big deal right? except when you have a claim they want another original receipt and the entire original lock packaging with the UPC code attached! HUH? Then they want a police report, great except most major cities won't come out to do a police report for a stolen bike. Then they want a slew of photos taken at the same time of day the bike was stolen showing the crime scene, what the lock/bike was locked up to, the undisturbed lock (how can you leave this till the next day to get a camera is a bit interesting), what they will decide from those pics if the crime scene was well lit with lots of pedestrian traffic. They want the original broken lock sent back. The lock warranty is only good for 2 years at which point you have to send money to reactivate it, then after that 2 year period you have to buy another lock and start over. When Krypto was questioned by a major bicycling magazine about how much money have they paid out in claims over the last 5 years Kypto's response was that it was a corporate secret...code for very little if any money has ever been paid out. Not sure about the other companies and their warranties, but I feel pretty confident that the others are close to the same thing as Krypto. There was a forum post once a long time ago on a different forum asking who had ever had their bike stolen while locked and if the warranty ever paid, several had their bikes stolen and none were able to get the insurance money.