this is completely subjective, in that it can only mean what do i myself find difficult. i found EGA a very difficult math book to read (too long, too abstract), and I find Russell Whitehead to be a book of logic not mathematics. Euclid is very easy and clear, although old. I like Riemann's works, although many people have found them impenetrable for decades. I like Dieudonne's Foundations of modern analysis, and spivak's calculus on mNIFOLDS, although not all do. baby Rudin is easy to read but hard for me to get any benefit from. all physics books are hard for me to read for the reason given by David Kazhdan(?) "physics has wonderful theorems, unfortunately there are no definitions".
you also need to define what you mean by difficult. does that mean which text is harder to plow through 10 pages of in a certain amount of time? or which is harder to learn something from? I once spent 3 hours struggling with a few pages of a research paper by Zariski and was very discouraged at my rate of progress in terms of number of pages. however, when i returned to class the next day i answered literally every question on that topic from my profesor until he told me to be quiet since i "obviously know the subject cold." so that research paper was much easier to read in the sense of how much insight can one gain per hour say than baby rudin.