Mansfield Park
Let me just come out and say it: Fanny, the protagonist of Mansfield Park, is hard to like. She shrinks, she shies, she sighs, and she never seems able to stand up for herself. She's so very good, but so very timid.
It wasn't until I read Mansfield Park for a second time that I began to like Fanny. And it was only recently (after so many readings that I've lost count) that I think I finally understand her and what Jane Austen was getting at.
The point with Fanny is that she's got incorruptible morals. Any opposition she makes is certainly not for selfish reasons (she will avoid conflict whenever possible), but when push comes to shove she won't give way on a moral point. I can't remember seeing a heroine like her in any other book I've read - a noisy defense of morality is more common, and certainly more in tune with modern sensibilities.
I'm still surprised at what a hold Austen has on modern readers, especially a book like Mansfield Park which is so divorced from modern concepts of behavior. But far more intelligent, educated women like Austen than an author who wrote so long ago has any right to expect.
I think Austen's books are loved for two reasons - her characters seem real and she constructs a perfectly rational world. I like the characters because they foul things up and make fools out of themselves and have to learn from their mistakes. But the perfect, unrelenting, rational story line (with a satiric bite) rolls on, dragging the heroines with it, to a perfect harmonious ending in marriage. I think this rational construction of a deeply irrational world is what attracts a lot of people - we know the world isn't like this, but it would be nice if everyone really did get exactly what they deserve (especially as we all know that we're the ones who deserve the happy ending).
Mansfield Park is almost frightening in the degree to which everyone gets exactly what they deserve (though modern readers might find some of the sentences too harsh). It is undoubtedly one of the harder Austen books to read - if you're looking for an introduction to Austen read Sense and Sensibility or, better yet, Pride and Prejudice. But if you've read Austen before and want a perfectly ordered ending written by a mature author, read Mansfield Park.