Proust. Just impossible to read. It really is like drowning.
The Divine Comedy. Nope, sorry, not for me. I just about made it to the end of the Inferno.
Portnoy's Complaint. Vile book with the most loathsome central character in all literature. Isn't funny either.
I also struggled with Shakespeare's comedies. I have read all the plays (it's my big boast), but Shakespeare isn't funny. I don't care what people like Harold Bloom say, comedy doesn't age well. Give me P. G. Wodehouse or Douglas Adams or Evelyn Waugh any day.
Howard's End. I'm sorry to say this, because A Room with a View is my favourite film. I also like Forster's other stuff. But I found HE slow and dull. Aldous Huxley, Oscar Wilde and Virginia Woolf cover similar territory, and do it more enjoyably. Maybe I wasn't in the right place when I read it.
Oh and Samuel Beckett. Ugghhh...we get it Sam, life's horrible and meaningless. You already said that in your last play, and the one before that, and the one before that.