Pratchett and Donaldson and Banks
I have read the vast majority of the Discworld series multiple times.
A few of the Culture novels by Iain M. Banks have been re-read. The Player of Games at least three times.
The Silmarillion can engross me for hours even though I read it through 5 times and slogged through the Christopher Tolkein dissections of the history of writing it.
I read through the first and second Chronicles of Thomas Covenant three times each. I've almost got a hankering to do it again, but I'm way behind on catching up last couple years of Banks, Pratchett, Gaiman and more.