Best Video Camera for Weddings, and Conferences?
The Canon is the only one of the three that is a camcorder. DO NOT buy a still camera for professional video.
Define "best". The Canon C100 is the only one designed to capture video. Assuming you are buying new, with a lens, that's ~$3k. At least consider a camcorder like the Sony PXW-Z190. It will provide a much wider poor-light window with the 3 sensor imaging system. The other two on your short list are designed to capture still images and happen to capture video as a convenience feature. Beware overheating issues published in their manuals available for download from the manufacturer's websites.
Prior providing $0.02, I did a search for "Canon C100 Mark ii vs GH5." Based on what owners of these cameras had to say, the GH5 is probably going to be the better option. However, you need to consider the glass that's available for these two very capable cameras. Are there any specific lenses that you may want to use that is only available in the MFT or EF mounts? Will you need a fisheye for ultra-wide establishing shots or maybe just because of the distinctive look that Canon's 8-15mm provides? Will you need 4K video now or sometime in the future? If so, then skip the C100. I would probably say that for a fraction of the price of either of the three cameras, you should seriously consider renting them and doing your own tests. You will discover what it's actually like to use these cameras. Everyone always just looks at specs, which is fine. But, what never seems to get any attention is what it's actually like to use these cameras out in the real world. You may find that one camera provides a better work flow based upon how you use it. Based upon specs alone, the GH5 is what I'd pick even though I always tell people to get a real camcorder. However, I've never used any of the three cameras, so I'm basing my opinion purely on specs and what actual owners have stated.
Right okay the Panasonic and Sony are not even camcorders.
And what prices do you pay for them