Very few books have I started but not finished. I read fast and can find some entertainment in most things. I can go through 3 or 4 books in a week's holiday.
Wuthering Heights, for a school book report assignment. Turgid rubbish, I only got 1/3 of it read. In fact I think I discovered the word turgid around this time, probably for this very report! OK I was a schoolboy and might have more luck now but frankly I'll just as soon give it a miss. Hundreds of millions of books have been published since the printing press was invented so I can afford to be more picky.
The White Queen (Philippa Gregory), given me by a neighbour, one of the freebies for World Book Day. Couldn't finish it. I don't think it was badly written per se but it was just Mills and Boon romantic nonsense in a historical setting. I love historical fiction but more of a Bernard Cornwell or Conn Iggulden man.
I love LOTR, Dune, Pratchett and many others that folks have mentioned on this thread. Even enjoy Jack Reacher if I'm in the mood for fairly mindless action thrillers. Currently going through the Song of Ice and Fire series, on book 4. Getting a little bored of some story arcs but then others pick up the pace a bit.