I don't have an e-book reader yet, and probably won't for some time; I just use my laptop and Calibre. The majority of the e-books I have (and will be purchasing for some time) are a) Baen free books or b) fanfic downloaded from AO3 and ff.net or c) classics downloaded from Project Gutenberg. When I do get around to buying an ebook reader, I obviously have no idea what kind of ebook reader I will be getting. So there's a good chance I'll have to either convert using Calibre or go back and re-download all my ebooks.

So the question is this: what format should I be downloading the ebooks in, that would either be a format I would likely use or easily convertible into whatever format I end up needing?