I've read Ulysses and Infinite Jest (the latter multiple times), all of Samuel Becketts novels, and the complete works of Italo Calvino and Georges Perec.

I maintain that although the prose was much much easier, 120 Days of Sodom was the hardest to pick back up after putting down. It's so tedious and repetitive but also about coprophagy, pedophilia and extremely detailed gore, with 0 plot or characterisation. I feel no sense of achievement for having read it and my life is worse for having done so.