Seriously... same as asking favorite 'song', or 'movie'... or ANYthing.
Most books I have re-read are 'favorites'.... so here are a few I've read more than once... some several times. List is very partial, obviously.
The Lord Of The Rings
Lonesome Dove
Rendezvous With Rama
Foucaults Pendulum
Confessions
Watership Down
The Bible
The Brothers Karamazov
Mere Christinaity
The Exorcist
Calico Palace (guilty pleasure)
Many, Many more... but who cares?
I would like to mention a recent read... only read it once because I JUST read it. I was wanting to read a western... been a while, so I did a search for the best western novels ever written and this one came up.... so I read it, and it was incredible. NOT very cheery... at all, but so well written. "The Big Sky" by A.B. Guthrie Jr.