I've re-read The Belgariad series, by David Eddings, so many times I've lost count. Easily once a year, every year, since I was in my mid-teens (I'm in my late thirties). It is most firmly in the territory of "comfort reading" for me these days.

Dune, by Frank Herbert, comes a close second. I've read that at least a dozen times or more. It took me ages to get it. My dad had a copy, and I'd tried to read it probably 4 or 5 times, never getting far. Then I watched the David Lynch movie, went back to it and just devoured the book in a couple of days. Even though the two are wildly different, something about the movie gave me that kick I needed to get me in to the book.

I'm really not one for re-reading books all that much. Most books I read once. The ones I do re-read, I've read maybe 2 or 3 times.