For books, it's my Kindle Oasis - if you are a serious reader, nothing beats a dedicated, e-ink lightweight e-reader. More than one hour on an backlit LCD screen, I will get eye strain, plus my iPad (Pro 9.7) is still quite heavy.
For magazines and rich media (lots of graphics and links), the iPad is the best.
I have the original Kindle, the original Fire, and the PaperWhite but prefer the Air 2. I bought the PaperWhite for my wife but she prefers the original, then I took it, then I got Air 2 and have stuck with that.