Not sure I'd put Karamazov on this list. It's long, and there are a lot of characters to keep track of, but Dostoyevsky was a damn good storyteller (and not so preachily moralistic as Tolstoy), so that made it a good ride.
I'm glad to finally find out what Ulysses is about. I thought Portrait of the Artist as a Young Man was a bit of a slog, but still great writing. I'll put this one on my list, along with the others.
My idea of a hard read is The Sound and the Fury by Faulkner. Stream of consciousness all the way, and it takes a good while just to figure out what's going on. Took me 4 tries to make it past the first chapter.