My answer is "whatever device the book is on."
I'm not a fan of the three-steps-to-ebook-process (download, convert, upload). I'd rather download directly from/to my ebook reader. I read Kindle books on a Kindle, Sony/Baen/etc books on a Sony (T1), and everything else either on my Sony (505) or one of my iOS devices (phone or tablet.) I'm pretty even across the board with what I use for my reading.
Therefore, I have no preference (and no option in the poll. I guess I'm not the poll target.
)