It will really depend on what you prefer, but here are some recommendations based on my experience...
Terrible. Absolutely worthless. In order to see traffic behind you, need to have you head at just the right angle. You end up having to tilt your head side-to-side in order to get a good view of cars behind you. I also tend to be pretty rough with my helmets when I am not wearing them (strapped to backpack, hooked on bike frame, regularly dropped, etc), and the mirror just ended getting beat up.
I imagine that glasses-mounted-mirrors are the same, by I have not used them.
Drop bar mounted
I used one of these once. It was ok, but you need to lower your head to check traffic behind you. It's easier to tilt your head to the side to check traffic. I do not recommend.
Side handlebar mirrors
I have not used these, but on a cruiser/bike with relaxed riding position, they might be nice. In theory, you could just move your eyes to the side to check the mirror. However, you run the risk of getting these knocked off by other cyclists, pedestrians, car mirrors, etc.
In my opinion, the best method for being aware of traffic and cyclists behind you is to
evolve highly specialized ears over 100s of years, allowing you to pinpoint the exact location of other road users just keep your ears open, and cautiously glance behind you from time to time. You don't have to worry about the depth perception problem, and you don't have to worry about having blind spots in your mirrors.