How to choose the best bike lock in 2017 - The Best Bike Lock

This is a really good read. Basically, stealing bikes is like stealing anything else: if someone is really determined to steal something, they can and will!

If you only park in low risk areas for short periods of time, you don't need a big honking lock. If you are going to park over night in an unsafe area, you'll something more substantial. Avoiding locks that can be cut with easily concealable pair of small bolt cutters is worth the investment. A lock that requires a 3 ft long set of cable cutters is going to deter more people.

There are some great video reviews on youtube where people show what it takes to cut various locks/chains.