First on my list is Zelda Pinwheel's version of Three Men in a Boat.
https://www.mobileread.com/forums/showthread.php?t=48377
I came across this when I was trying out freebies and mostly finding junk with typewriter quotes, weird page breaks and the like, so it was a cool, shady oasis in a very big desert. I have tried it across a variety of PC-based epub readers, as well as the Nook. Allowing for differences in screen size, the pages essentially look the same on every reader I have - like they came directly from a nicely formatted paper book. She marked it as an experiment "to see how far i could push the epub format," and I'd say it worked out very well. This and Elizabeth Castro's EPUB Straight to the Point, with its attached samples of Walden, are the two things I'm using to study epub formatting.