I'm quite sure the OP was referring to actual dedicated dash cams currently available in the marketplace when he launched this thread and asked his question but the high end professional digital video cameras posted to this thread make for an interesting discussion, so what the hell, I have another one to feature for those who may not be aware of this growing trend towards miniaturization in high end cinematography.
My post here was prompted when I happened to take a closer look and noticed that one of the cameras posted by
@thancam above, somewhat obscured by all the mounting paraphernalia is the
Black Magic Pocket Cinema Camera which measures 5.04 inches x 1.5 inches x 2.6 inches. Black Magic produces some of the more unique professional cinema products on the market and has specialized in highly capable, increasingly small devices to complement their larger offerings.
Pocket Cinema Camera
One of their more recent additions is called the
Black Magic Micro Cinema Camera, measuring only 2.74 inches x 2.54 inches x 3.25 inches (sans lens)
The camera uses a Super 16 sensor (12.48mm x 7.02mm) and can achieve 13 stops of dynamic range! - 1080P @ 23.98 to 60 fps
It accepts SDXC and SDHC memory cards, MFT lenses, has top and bottom tripod mounting options and a full spectrum of high end cinema controls and features.
At $995 (not including lens) it might be considered the ultimate action camera! (and for the purpose of the thread, perhaps the ultimate dash camera)
A 4K "studio" version is available as well.
Pretty cool! So to answer the question, I think this is what I would buy if cost and installation wasn't an issue.
If we are ever to start seeing dash cams manufactured with larger sensors like the Super 16 or MFTs the Black Magic Micro Cinema is a good indicator of about what they might look like.