I've owned all of the tablets you're considering except the newest generation Fire HDX and Nexus 7. In my opinion, for a combination of price and features, you can't beat the Nook HD or HD+.
You may be interested in Nate Hoffelder's newly updated review of the HD+:
http://www.the-digital-reader.com/20.../#.UsWA1TaA3OE
As he mentions, rooting it or running a dual-boot Android OS on an SD card makes it much more versatile. I use this dual-boot AFN card in mine:
http://www.androidfornook.com/product-info
The Kindle for Android app works great with Kindle books and even without rooting it, you have access to GooglePlay apps. Also, I far prefer the 9" screen to my 7" Kindle Fires and Nexus 7.