If reading is your primary goal (meaning no video or productivity apps/games, etc) than a kindle is hands down the best option. They are cheap, very easy on the eyes, and the battery life is incredible.
The iPad is awesome, but as has been stated it's not the best device to read on. I have a lot of books on my iPad and I kind of regret purchasing them. I am really hung up multi-purpose devices and I thought the iPad was going to make the cut for reading, but in all honesty the kindle blows it away. You can't beat e-ink right now, it's not even up for debate.
I think the Kindle fire is an absolute piece of garbage that will go down in flames (pun definitely intended). However, the fire helped to drive down the prices on the kindle touch so now is the time to pounce if you were on the fence!