I've come to prefer the EPUB format as content sizes to fit the screen optimally, and zooming in/out changes the font size but doesn't require you to pan & scroll. I rarely need to annotate ebooks, so annotation options aren't a factor. Simply highlight text works just fine in EPUB. I used to prefer PDF because the page looks more faithful to a printed page in terms of formatting, but the zoom issues were a big negative.

As for my preferred reader for a "public domain classic book", I prefer the eInk Reader, not an LCD app.