I don't know how many times I have tried to read Delany's Dhalgren but it is at least a dozen and I just cannot make it past 20 pages or so. I've read Paradise Lost and, of course, Moby Dick. Milton is just a PITA and odd so I agree there about Paradise Lost. Read as a class though it was at least a bit interesting. I actually quite liked Moby Dick and did not find it a tough read as a pre-teen.
Hands down impossible reads for me would be Homer. Illiad & Odyssey are, to me, totally incomprehensible to the point of inducing the need for therapy.