Yeah, this is always a concern. Bike thieves unfortunately have the edge at this point, since they can make quick work of most U-locks with an angle grinder. Following my historic rule of thumb that I will spend around 10% of the cost of the bike on a lock, I settled on a LiteLok X1 for my current ebike, which purports to have some resistance to grinders?

Most of the time though, I'm riding to and from work, and have indoor storage at either end. When running errands, I have different levels of paranoia that trigger different behaviours. If I'm just hopping into a fast food joint and sitting where I can see the bike, I might only self-lock it with a chain lock I kept around from my older bike. Max theft deterrence would entail both locks + remove battery + remove seat. But I've never left my bike parked out overnight anywhere. That's just asking for it.

My strategy has been 1) Ride affordable bikes 2) Use at least 1 quality lock and 3) be strategic about how/where you park or store your bike. Be sure to lock up in highly visible areas covered by cameras if you can and lock to objects that would be difficult to cut through. Most thieves use a battery powered cutting wheel these days.

The locks I use vary depending on the bike/situation but typically, I am using a Skunklock Chemical, Foldylock Forever, or Hiplok Gold (or some combination thereof) and also sometimes a cable and cheaper Kryptonite U-lock for added security. Most of my bikes also fold, so I typically bring them in with me both at home and work (sometimes even into stores - my Portola ebike and Priority Folder bicycle both fit under a shopping cart).

If you have a nice e-bike, you should also get insurance, add trackers, a motion alarm, and brake locks. A good cut resistant u-lock like the Litelock X3 would be a good choice for a primary lock.

I use a thick bike chain lock. Kryptonite 1016 Evolution specifically. It's also fairly long at 5.25ft/1.6m, which allows me to chain it around small trees. After reading some other replies I think I'll be adding a u-lock too. I also always lock it up in a visible area.

It's about sufficient deterrence, I think. Which is situational. The longest I ever need to leave my bike unattended is an hour. But usually less. I feel my security is sufficient as it's difficult enough to defeat that a potential thief couldn't do so without drawing unwanted attention. But I wouldn't be comfortable with that as a solution for say an 8 hour stretch of time.