My favourites are:
Greg Howe: Just go hear this guy play La Villa Strangiato and you'll see what I'm talking about.
Allan Holdsworth: At times, it sounds almost like a synth.
Dimebag Darrel: In a class by himself. Mind blowing. Not just his shredding, but his rhythm is a rock wall of power.
Martin Taylor: The cleanest of the clean. Listening to him play solo is like meditation.