Apart from the late individuals already mentioned, I feel inclined to add Criss Oliva, former guitarist of Savatage, to the list. Today, he is still one of the most underrated guitar-players in metal, and his manner of death was rather disheartening, especially since I'm a resident of Florida. For those unaware, he was killed by a speeding drunk-driver while en route to a concert in my homestate. His wife, who was in the passenger seat, survived to endure the grim reality of the tragedy. NOBODY could mimic or emulate Criss's unique style of playing, nobody.
R.I.P.