Umberto Eco is my top of the list of writer who I did struggle in reading.
He is a learned man, and he did try to tell you about it. It is not a page turning, and reading some of his work made me feel like I've accomplished something.
Another one that I've difficulties is with José Saramago. I'm no good at punctuation, but no punctuation is like swimming through pages and pages of words without a stop.
With that said, I like them. They are really good writers.