I use a Hafny bar-end mirror. Very good optics, easy to install and mine never gets out of position, despite me hitting it once in a great while with my knee.
With that said, the best bike accessory purchase I ever made has been my Varia rear light/radar unit. Working with my Garmin Edge, it detects and displays on the Garmin’s screen all vehicles (and bikes) that are approaching me from behind. I never ride without it.
The combination of the Hafny & Varia products give me great piece of mind and I consider both of them money well spent.