I have three cameras that I have used to document my rides since I started cycling. The first camera is a Contour+2 from Contour but I can't tell from their web site if they are still in business or not. That's a shame really because the Contour camera line has, in my opinion, the best form factor of any action camera on the market. It's just that their tech is a little dated as the camera only shoots at best 1080HD and can't compete with newer 4K cameras from GoPro. Contour's other pluses are their mounting system (the camera has rails and so has secure mounts for a variety of applications, the battery is swappable and holds a charge for about one and a half hours, and the lens which produces very nice images even though limited to 1080, and there is a mini mic jack on the bottom of the unit for an external microphone. Contour does, or did, make Storyteller software for their cameras from both Mac and PC which allowed you to import video from the camera and also adjust the camera's settings.
My other camera is one from iON - an Air Pro 3, but I don't use it much mostly because of the bike mount. The handle bar mount doesn't hold the camera securely and so the camera rattles which produces shakey images and the internal microphone picks up all the noise of the camera vibration in the mount. So what you end up here is a bunch of plastic on plastic clatter while watching video which may induce motion sickness. Another drawback to this camera is its poor battery life, less than an hour in my experience. The battery is internal and not user replaceable. To recharge the camera you have to remove the WiFi pod from the back and connect it to micro USB cable. There was no software included with my iON camera bundle but you can download an app for your smartphone or tablet. You then have to connect to the camera's WiFi network where you can change some of the settings and view video.
Lastly, my newest addition is a Garmin VIRB XE camera. Like the Contour+2, the VIRB has built in GPS and a user swappable battery. The bike mount holds the camera securely and the microphone is pretty good at picking up sounds, but better at capturing road noise. If you want to be heard on the video, buy a bluetooth mic or dub it in later. The VIRB is also a 1080 HD camera, not a 4K, and I don't think the images are as sharp as the Contour but noticeably better than the iON. You can also control the VIRB from your Garmin Edge cycling computer, but I've noticed that I often have to force my Edge 1000 to look for the VIRB since it doesn't always auto detect it. If you use the Garmin computer/camera mount the VIRB will be upside down, which isn't a problem as far as video or photos are concerned since the camera can auto detect its orientation but it does make it difficult to read the display on top of the camera which is now facing the ground. I haven't found a way to display any fields from the VIRB on my Edge, though there may be some way to do that. Battery life on my unit has been about 45 minutes so I purchased a spare battery so I can swap them out on longer rides. Where the Garmin shines is with the included VIRB Edit software which allows you to not only import clips, but edit your video and include overlays of data fields from your bike computer. You can also add music and do some basic effects. Of course, if you want to do a lot of high end stuff, you can export the video and import it into iMovie or Final Cut.
So in my opinion the Garmin VIRB XE ranks first due to its secure mounting, built in GPS, swappable battery, and included editing software. Next would be the Contour+2 with its one hour plus battery life, GPS, and Storyteller software though be warned that there is no telling how long Contour will be in business. Lastly, the iON camera which with the bike handlebar mount is pretty much unusable, thirty to forty-five minute battery life, and no software.