But the real answer... I have seen and used several phones and a couple of tablets, and I have never seen an LCD device that does well in sunlight.
That is why people who want to read eBooks in the sun get devices like the Kindle Paper White with ePaper displays instead of lcd.
In my experience, the trick for car mounting is less about the device, and more about finding a location and angle that works for you without blocking your vision of the road. I have found that for me, even though I have Copilot loaded on my Nexus 7, I prefer using my phone in the car because I can set it on the dash in front of my tach, and I can see it perfectly.