Part I: A Clear Definition Between Good And Evil
I'm all for a good book that can blur the lines, but honestly, I haven't found one that I'd call a "classic" that can do it well. All of the books that I would call my favorites, the ones that I want to read again and again, clearly define who the good guys are and who the bad guys are. Some minor characters can waffle a bit and the main characters obviously make mistakes, but the protagonists and antagonists know their roles and stick to them.
Harry Potter - Harry Potter is a classic example. Good guys: Harry, Ron, Hermione. Epic bad guy: Voldemort. There is no question throughout the entire series where these two parties fall. You know who the bad guy is and you know whose job it is to take him down.
Lord of the Rings - Again, a very clear definition. Good guys: The Fellowship, the elves, the Rohirrim, you name it. There are a lot of good guys in this series, and most of them are awesome. Epic bad guys: Sauron, Saruman, the orcs, pretty much anything associated with Mordor.
Star Wars - Another perfect example. Good guys: The Jedi, the Rebels, Han, etc. Bad guys: The Sith, the Empire
Clear cut sides give us something to cheer for without fear of hitching our ponies to the bad guy in disguise.
I honestly can't think of any really timeless fantasy books where there isn't an obvious "good vs bad" battle. I can think of characters who don't necessarily fall on one side or the other (Snape and Boromir for example), but no books. I initially thought of Frankenstein here, but decided against it in the end as most of the notion of the monster being bad comes from the movies, not the book itself.
One of my most favorite movie scenes ever
How many of your favorite books follow this rule? Can you think of any exceptions?