And the Winner is EPUB!

https://fandom.ink/@unspeakablehorror/110838084253226040

This was a very helpful poll for me! Got more responses than I expected to. While my particular choice of words and the audience this was presented to could have skewed the results, I do suspect that a lot of people do like EPUBs and a small but significant percentage prefer PDFs.

I suspect that a lot more people are willing to read books in HTML than this poll indicates because I think many people are willing to read books online even if that's not a format they generally prefer to download. I also think the lack of download preference for html is partly due to there generally not being an option to download it on mobile (which could be given by a webmaster) and partly due to it often downloading in inconvenient ways, such as in multiple files, which can also be remedied.