My first question, what do you actually want to do with it? The Nexus 7 and iPad offer totally different things. The Kindle fire isn't even on par with the other two tablets mentioned (I'm not biased I just don't think they should be compared).
Well if you simply want to read books and have a gentle surf then the Kindle is probably for you. If you have the money (I'm not saying you don't) then I'd probably go with the Nexus 7. The screen is immense, although it's not Retina, but it's still great. The Nexus is a real power house and the best part about it is the price. You can also get a Kindle app which syncs all your books x-device anyway