I used the iP7+, V20, and S7 edge as a dash cams a couple times just for fun. All at full resolution auto.
The iP7+ had the best stabilization, best true to life colors, but too many stutters with playback and poor focus in light transitions.
V20 had the clearest video, almost no stutters, but in darker light like very cloudy conditions it has noticeable noise.
S7 edge had the best focus, no stutters, but color saturation was noticeable.
I haven't tied my S8+ yet.