Personally, the device that is most fun would have to be a tie between My PDA Phone and my Sony PRS 500.
The Phone allowed me to always have a few books in my pocket (Who goes out without a phone), so that whenever I had some dead time, I could get some reading in. And while I did continue carrying books in my backpack, having the phone meant that I was never without distraction
The Sony changed that even further, that I no longer needed to carry any books with me. Also while I might have used my PDA when I didn't have other options, I would still prefer a Proper book or my laptop before choosing to read on it, if possible.
With the arrival of the PRS 500, I started to prefer it to all other options.
I actually did't realise how much I liked it until I gave it away, and so finally bit the bullet to order a Cybook.
After all as I haven't got it yet, I cant really say it is the most Fun......Yet