The Harry Potter series by J.K. Rowling. Easy answer. *g*
Of course there are plenty of books I've reread over five times over the years. reread V.C. Andrews novels every now and again, I probably have read Flowers in the Attic many times since I first read it years ago. Sometimes, you just have to relive certain books that you love, especially if they made an impact on you. You know what's really interesting, is that there are people out there that think it's weird or pointless to reread books you've already read. Whatever, I say they're missing out on half the fun. Because sometimes you reread a book you read years ago and you get a totally different perspective, or you catch things you didn't before, make certain connections, etc. I mean, rereading Harry Potter was a vital part once the saga was near completion, because there were clues hidden in the previous books. So yeah, rereading books several times is awesome.
And I just rambled, lol.