"Best" depends on what you want to do with the camera.
But anyway, here are some ideas.
I no longer ride with the camera mounted on the helmet. (For safety and comfort reasons.)
If you want to mount the camera on the helmet, chin bar mount is your best bet.
Chin bar mount will give you the most natural POV shot.
Side mount is my least favorite.
You will always get a part of your helmet in the frame.
And this is the least comfortable option.
Mounting the camera on top of your helmet ...
... will make you look stupid, but, it gives you pretty good POV image.
Depending on the footage, helmet top mount is better than chin bar mount.
For example, when you turn your head (which you will frequently do on the street), the camera will also pan.
With the helmet top mount, because of the camera being close to (or in-line with) the axis of rotation (of your head, which is your neck), the image of camera panning is a lot more natural.
With the chin bar mount, frame will swing abruptly.
Side mount is the worst.
The downside of the top mount is that the camera position is just too high (to be the rider's POV).
Also, it will make the video looking like you are riding a lot slower.
Look at the video.
