Nook is essentially out of business. As are most others (Sony, Kobo, etc) and of course their problem (same with the Kindle) is that they are locked to one book-selling store and no other.
This essentially leaves two options: a dedicated e-reader (of which pretty much the only one left standing is the Kindle Paperwhite), or a tablet (iPad/iPad mini, Nexus, Note, Kindle Fire, and about a million gone-tomorrow lesser brands)
The Paperwhite is a fine dedicated e-reader that can (barely) manage a few other things, but is locked to Amazon's store. This is the same problem with the Kindle Fire, though as a full tablet its more versatile than the Paperwhite of course.
So this leaves "real" tablets like the Note, the Nexus, the iPad. To the best of my knowledge, apps available for these devices allow you to buy and read (separately within the apps) books from basically anywhere, starting with the public library and going through every proprietary or open "e-bookstore," including Amazon.
This allows one the greatest versatility and lowest risk on investment, so to me a tablet (in addition to the millions of other features it offers) is a great choice for e-reading except where access to electricity is extremely scarce (the Paperwhite's primary distinguishing feature -- e-ink uses far less power than tablets).
So then it comes down to "what's the best tablet?" There is no ONE answer, because there are varying factors, but the iPad, Note and Nexus are always at the top of the list. It's telling that the senior editor of androidpolice.com picked the iPad Air 2 over any Android tablet, in my opinion.
For my own personal use, I need something that works with the Apple eco-system (since that has the most apps and most tablet-specific apps), has one of the best displays, the best customer service, the best security, and (again, for my purposes) is a full-size tablet rather than a mini. For me, that means the iPad, specifically the iPad Air 2. I adore it, and read from it constantly (though I also write a lot on it, using a keyboard case). Your mileage, as they say, may vary.