As noted, any lock can be defeated. The goal is to use a lock sufficiently tough so that it will:
1. Take too much time.
or:
2. Attract too much attention.

So, it's not only what you use to secure your bike, it's where and to what. Although angle grinders and huge bolt cutters are effective, they require a bit of privacy... Most thieves don't want to carry around a three-foot set of bolt cutters that weighs 25 pounds, and grinders make a lot of racket and throw sparks all over.
In fact, in my (considerable) experience, the vast majority of bikes that are stolen are mid-level mountain bikes that are secured with cables.
The cables are easily cut with a very small bolt cutter, and the bikes can be quickly pawned or sold.

High-end bikes are harder to move; pawn shops are generally not interested, and the local crack dealer isn't either.
In certain areas, there is sophisticated traffic in high-end machinery; even to the extent of re-stamping or altering serial numbers and such.
That's unusual, though.
We give the students a discounted Kryptonite lock, and over the many years we've been doing this I can count on one hand how many of these have been defeated.
The many bikes we do get stolen are almost all secured with cable locks.