I can't seem to choose between the Mamiya 6 and the Fuji GF670 so I've kept both. I did let go of the Fuji GS645 in order to finance the GF670.

As you mentioned, the Mamiya 6 has the best ergonomics, and the 50mm lens is absolutely top notch, although the others are great too.
The GF670 is superb in every regard. I just love it. One obvious difference between it and the Mamiya 6 is that the Fuji viewfinder is much brighter. And while the GF is bigger than the GS645, the difference is not as great as one might imagine. Take a look at this article to compare the size of these two folders. It's in Japanese but the photos are very helpful even if you don't read Japanese.

I've never owned the Bronica RF645 but have read several reports that it is prone to mechanical problems.