Unfortunately, neither Amazon nor Barnes and Noble, maker of the Nook, report sales figures so it's a little hard to get accurate information. But according to research group International Data Corporation (IDC), the Kindle has a market share of 51.7% and the Nook comes in at 21.2% (though Barnes and Noble has said its share is closer to 27%). So no matter how you slice it, the Kindle is by far the market leader.
And, of course, we have the wild card of the iPad, which isn't typically categorized as an e-reader, though it is certainly used that way by many people. The important thing to remember is that sales of e-books are headed up.
So what's the best e-reader? Doc points you to a great summary review over at cnet by John Falcone. It's a very extensive look at the various devices and a good primer on e-books as well.