The Illiad. As the foundation of Western literature, it may have received the greatest amount of hype that it is possible to receive.
I had attempted to read it and The Odyssey when I was much younger, in middle school, maybe too young, and failed miserably. But as it is always with cultural touchstones, references to it are inescapable, and a few years ago, in my mid 50s I overcame the reluctance accumulated in the intervening time and set out to read the Samuel Butler prose translation of 1898. Most of the translations I'd previously approached were in verse, which is in some ways its own hill to climb when one is customarily a prose reader, so this seemed perfect.
And so it was, and I was bowled over. It was a mind bending experience for me the likes of which I experience much to infrequently as age and experience take their toll on the novelties of youth. I can't say the last work I read I experienced as electrically. For all the stiltedness of epic story telling, the personalities of Agamemnon, Achilles, Zeus, et al are both vivid and convincing; the violence of battle is horrible, electric, and wierdly beautiful in a way that will resonate recognizably with fans Sam Peckinpaugh or Hong Kong action movies. Though the characterizations are far more stereotyped, as befits the age in which it was produced, than modern readers are accustomed to, they still evince a polish that rises above conventional story telling into true literature. Worth every ounce of effort you expend to summit this one.