What mirror is best really depends on personal preference. You need to try a variety of types to decide what works best for you.
I've used a dozen or more different mirrors during my riding years. In my case, I prefer the helmet or eyeglass mounted models. They allow adjusting the field of view with a slight head movement which you can't with a bar mount. I have 3 bikes so I would have to put bar mounts on all 3. Since I always wear a helmet, I've tried several helmet types. Unlike eyeglass mounts, the mirror comes off with the helmet and is one less thing to fool with. Some helmet mounts do shift easily as others have said. I eventually found this one from EVT:
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It uses Velcro and / or zip ties to fasten to the helmet and is rock solid. It takes a significant jolt to knock it out of alignment. At $40, it's a bit pricey but worth the expense IMO.