I have a Paperwhite and a Touch with a lighted cover and I had a Kindle Keyboard at one time, although not with the cover. I think the Paperwhite is far the easiest to read with because of the light.
I don't like to use it in the dark. I don't really like reading in the dark. I didn't like that with the Touch and it's cover, either. I just like to use the Kindle's light to whiten the background.
In my opinion, if the screen glows, the light is turned up too high. I've played with this quite a bit and I think as the light goes from nearly off to brighter and brighter, contrast increases up to a certain point. When it begins to seem like it's glowing contrast begins to decrease. Where this point is depends on the ambient light.
I'm pretty old (sorry if that sounds like bragging
and I need good contrast and good light. With the Touch and the Keyboard I used to have to adjust my position and the lighting till it was just right. With the Paperwhite I still want decent light but I don't really worry about it. Just about any light will do.
Sitting on the front porch in sunlight, they're all great screens but in situation the Kindle Keyboard is the obvious winner. It is whiter in sunlight where the light doesn't help. But in sunlight the Paperwhite is still excellent and in any other situation it's by far the best.
Barry