It was hard deciding between four that I wanted to nominate, including the Decameron, Chaucer's Canterbury Tales and Plato, but I'm going with:
One Thousand and One Nights, aka Arabian Nights
It was written over centuries starting around the 10th and is quite large with many different versions. I'm not nominating any particular version or translation or length except that I'm sure we all will read an abridged version of some sort (the 'full' text is thousands of pages!); everyone can pick which they prefer but there are plenty available as ebooks in all price ranges (including free) and lengths. You can choose to read a short version of a few hundred pages or less, a medium version of around 400 pages or so, a longer (or much longer version), or pick a longer version you like and only read portions of your own choosing.
I don't have time to investigate further, but a quick Google search on best versions brought me these three links as some of the top results that I've only glanced over so far but seem to have some good info for choosing an edition/translation:
Annenberg Learner
Metafilter
Reddit
So far the Dawood looks tempting to me- has an ebook, lauded, fairly short for a version trying to be somewhat well-rounded, includes the frame story and earlier and later stories. The Haddawy also sounds good but is longer and in two editions, one including earlier stories and one including later, and includes less of the frame story. It sounds extraordinary but I don't think I've ever read any part of these stories, though I'm familiar with some of them through films and such.
PS- It may be a while (later today) before I have time to properly list all the nominations in the first post and begin the vote, so until I do nominations will remain open.