take-a-look helmet visor or glasses mounted one is in my opinion the most versatile one, easily mounted to various shaped mounting points, flat mirror-ie not convex so a very accurate depth perception/distance perception compared to wide angle convex mirrors, very tough, completely adjustable so can go on either side of helmet.

here is a review of it with video

Take-A-Look Mirror | TravellingTwo: Bicycle Touring Around The World

I have used mine for about 6 years and with all the banging, dropping and sticking my helmet in panniers or whatever that comes with everyday use, it is in exactly the same condition as new. I attach it to a helmet visor, the very convenient and easy "three prong" attachment system is easy to adjust to various thickness of mounting points and maintains its hold very very well.

complete winner from the day in, day out (year in, year out) point of view of both use and being very tough.