Mount Everest (9)
You might be skeptical that a poem about knights in shining armor and damsels in distress could really be that tricky, but Spenser's The Faerie Queene is up to a whole lot more than just some good old story-telling. Spenser intentionally wrote The Faerie Queene in archaic, out-of-date language, meaning that reading Spenser was strange even for someone from his own period.
Combine that with the fact that every canto is divided into sonnet forms of his own invention—and sonnets are not a poetic form typically associated with narrative story-telling—and you've got an amazingly daring and beautiful poem, but one that is 100% challenging. If you're familiar with other chivalric romances or long mythological stories, you won't be surprised to also learn that Spenser packs his narrative with lots and lots (and lots) of characters, many of whom are hard to keep straight and some of whom Spenser even randomly renames or gives multiple names to (gee thanks, Spenser!).
And for the final touch of challenge, The Faerie Queene is also written allegorically (check out Images, Symbols, and Allegory for more) so it can be unclear whether we're supposed to understand things as actually happening, representing something entirely different that's happening, or a little bit of both. This amazing poem is absolutely worth the effort, but just don't crack it open expecting your next beach read—crack it open expecting to have your mind cracked open.