Best is qualitative not quantitative. Both the newest versions of the Nook and the Kindle uses basically the same display, so it's more of a question of which interface you like better. I've used and at one point owned both.
Now that's for e-ink. Yes there are other brands, but please, you are either looking at the Nook simple touch (touch screen), or Kindle (the basic one, the touch, or the keyboard one)....
Now if you are talking about LED/LCD, you are more in tablet territory. Kindle Fire isn't out yet, the Nook Color is about to get a refresh, and of course there is the iPad...
But yeah, I'm guessing you are considering e-ink displays, in which case you need to play with both of the big boys and see which one you prefer.