First, decide if the screen must be backlit. If not, the cheapest Nook is your best bet. It's on sale right now for $59, with free shipping. That's as inexpensive as they come. The Kobo Mini is also $59. The Kobo Touch, with a 6-inch screen, is a bit bigger than the Mini and is priced at $99. But bear in mind that Kobo ebooks are generally more expensive than Nook ebooks.
Many people prefer a screen that is lighted, so you can read in the dark, or simply because a backlit screen is easier on the eyes. The Kindle Paperwhite is $119 with "special offers" that appear on the screen before you start reading, or $139 without them. I personally don't mind those offers - they disappear as soon as you go to the home page or to the book you have been reading. The Nook GlowLight is $119. It carries no advertising. Kobo offers the Glo ($129) and the Aura ($149) in this category.
All the backlit devices are excellent. You can read with the light at full strengh or reduce it a bit on any of them. Each has built-in Wi-Fi so you can connect, and then buy a book just by looking it up and entering "buy" (provided you have registered a valid credit card with the seller.) Books download immediately on all these devices with a good Wi-Fi connnection.
There is a slight difference in the dimensions and the weight of each basic ereader. I love the fact that I can hold any of them in one hand. They are perfect for taking a slew of books while traveling.
Kindle probably has the largest selection of books available in its proprietary format. It does have a strange way of displaying page numbers (some earlier versions did not show page numbers) as it also shows percentage of a book read, and, if you wish, it shows how much time you need to finish the book.
Barnes & Noble's Nook has a somewhat smaller collection. (A friend could not find an Italian-English dictionary for her Nook, for example.) But B&N holds frequent sales on ebooks and offers many free ones that are in the public domain. Amazon typically discounts most Kindle ebooks, so nearly all cost less than the Nook version. But the price spread often is less than one dollar.
With all these devices, you can borrow ebooks from local libraries that have an elibrary function. Amazon also offers a free loan of certain Kindle ebooks once a month to its Amazon Prime customers - although typically not current best-sellers. Kobo's strong point is that you can buy its ebooks through your local independent bookseller.
With any ereader, you can change the type font and the type size, as well as typeface and the space between lines. All will work for many hours before they need to be recharged. (It's best to turn off Wi-Fi for the longest charge.) None of these basic versions have the full-fledged browser or video viewing capability that each company's more expensive "tablets" provide.
The verdict: It is hard to go wrong no matter which ereader you choose.