This depends on what you need. If all you want is an affordable reader that does the job well, the basic Kindle is a reliable reading companion. All current Kindle readers now have a backlight, a touch screen and batteries that can last several weeks. If you want a mobile connection or a waterproof body, then you need to go for one of the more expensive options. The Kindle Paperwhite is more premium and waterproofed, and it strikes the best balance between quality and value.
The Kindle Paperwhite Signature Edition adds in some nice extra features like wireless charging like wireless charging and an auto-adjusting display, but they're hardly essentials. For the 4G connectivity and the latest tech, the Kindle Oasis is the best of the best, but you have to pay for it.