I have a Nexus 2013 (7"), Samsung Tab 3 (8") and a Kindle fire HD (7"). My husband has a Kindle fire (original) and an Ipad 3.
First, there is no reason to get the Kindle Fire family UNLESS you have prime and want to watch movies from Amazon or UNLESS you have a lot of software/music/movies that you already own from Amazon. I have owned all the Kindle Fire family up to this year, and I love the hardware and price point and (with a sideloaded launcher) you can have a good android user interface. But this year I decided not to buy another, for the following reasons.
1. I have no use for the movies (Fire's best feature), since my bandwidth is too expensive to download them more than about once a month.
2. Their software apps get updated VERY late. Anywhere from weeks to months after the i-store and google play store. I've seen as much as 8 months late. The software developers say this is because 1) developers develop for i-store first, and 2) Once Amazon receives an app from the developer, it takes weeks or months to review the app and post it to the store. Amazon is trying to incent developers to write more stuff for the Amazon store. They'd do better to upgrade their own approval process so that the apps get updated in a timely manner.
I do use a Kindle, but not a Kindle tablet. I use my Kindle ereader constantly.
I love my Samsung Tab 3. It's almost a year old now and it's the tablet I use most. The 8" screen size adds a surprising amount of real estate over the 7" models and still fits in my purse. The screen is not as high res as models that have come out later, but it is very very sharp and clear, and the colors are intense. The ALL the google play apps that I have examined, run on it (developers always suppord it because so many people have one). Updates come soon for apps. It gets OS upgrades later than my Nexus, but I don't really mind about that. I've been eyeing the Note 8, which is higher res and came out more recently, but I've decided to wait until 1st quarter next year when probably the next Tab will come out. Note has a high def stylus (Wacom) and you can write & draw on the screen with it. This is what you pay extra for, and I'm not interested in it. Though it is cool to play with in the store
Apple. Well, it's the bomb if you like Apple. My husband is VERY happy with his ipad and it's not even the latest model. Also, there are many app developers that stick to Apple, so if those apps are important to you, that's the way to go.
You didn't ask about the Nexus. It's great hardware, and bare Android. It gets OS updates at once (of course), and the price is good. For some reason it doesn't grab me as much as the Samsungs, but that's personal. In every other way it's just as good as the Samsungs and better value for what you get.
You should check out the app stores (google play and i-store) and see which one has more stuff that appeals to you. However good the hardware, it's the apps you spend time interacting with. I know that in my husband's hobby (strategy gaming), the i-store has many options that are not available for android. For the games I like best (resource based), there's tons on Android, and they're a whole lot cheaper than the i-store games. If you're not interested in gaming, the same still applies to other types of app.