First and foremost, Tolkien:
Lord of the Rings, 6 times so far,
   (but for The Hobbit, once is enough.)
The Silmarillion
The Lays of Beleriand
The Book of Unfinished Tales
The Book of Lost Tales
Leaf by Niggle
Sir Gawain and the Green Knight
(and at least two others, the titles of which my aged memory refuses to yield up.)

The Rubaiyat of Omar Khayyam, of course
The Rime of the Ancient Mariner
Wind in the Willows
Yes, and Watership Down

Zen and the Art of Motorcycle Maintenance

Beowulf

A Confederacy of Dunces 3 (or is it 4?) times so far

I just reread Master and Commander, as the beginning of rereading that whole cycle of great novels.

Those are the ones that spring fairly easily to mind.

Dave Oesterreich