The only real shredders mentioned above are pobably alex skolnick and marty friedman.
As for the others, I would never pay money to watch Zakk Wylde play an instrumental set. Although he did play some intersting stuff on two of Derek Sherinian's CD's he is certainly not a shredder, but his fame as Ozzy's sideshow makes him a guitar god. Has enyone seen or heard him play guitar solos during Ozzy concerts (ie Bodokan, or Live ad Loud, or actual live shows). It's crap. Yes he has great technique but certainly not G3 calliber.
For that matter, I also have a lot of respect for Dimebag (R.I.P.) and enjoyed his plaing but he would not strike me as the instrumental type to be on a G3.
Alexi Laiho is a ****in crackhead who does play some decent solos, and much better than many metal guitarists out there. If he got his **** together he could be one of metal's greatest young guns.
Therefore my list: (1) Friedman, (2) Skolick, and (3) .... maybe Laiho
Although Petrucci is pretty metal if you ask me and kicks all of the above's asses.