My teacher last year, in the one useful thing he did all year, gave me a list of all the books that have appeared on the Free Response section over the years. The three that have appeared the most are Invisible Man, Wuthering Heights, and Great Expectations. (I can send you the whole list if you'd like.)
Princeton Review also recommends Shakespeare's plays, as they tend to encompass any of the prompts the test throws at you.
But then again, I didn't read any of those and still got a 5 on the exam, so take the statistics with a grain of salt.
I only prepared to write about the books I actually genuinely liked (Death of a Salesman, Long Day's Journey into Night, The Metamorphosis, Pride and Prejudice) so I could write a genuine, passionate essay. Don't get too caught up in numbers or hearsay - as long as you have prepared a few books that you genuinely love that encompass a variety of themes, you will be A-OK.