Hmm...hard choice. It's between my mildly beaten dx rancho roc (it's even got my own artwork I did for the club stamped on it) and my champ Teerex-X. I'm going with the Rex-X. It's just so reliable for me. It's a moderately overstable disc...not so overstable that it won't glide and fly for me though. I like flexing it, I like hyzering it, I like bombing it on a slight turnover. I think it's the teebird of the long fliers.
I also like that you asked for our FAVORITE disc, not the most important disc or the most used disc. There's something to be said about your relationship with a disc. There's a trust factor with your favorite disc.
Esp Cyclone. Favorite of today- my new *Destroyer. It's flat, 171g, has sort of a thumbtrac where the rim meets the flight plate- these Destroyers fly 50ft+ farther than the domey rounded ones, and go effortlessly far sidearm. It's like a different disc.