Jon Oliva + Paul O'Neill: Savatage and TSO have written some of the best stories put to music, and these two are the primary reason.
Jim Matheos: Fates Warning has had some pretty damn good things to say throughout their careers, but just listen to the words in The Ivory Gate of Dreams or A Pleasant Shade of Grey to fully appreciate this lyrical master in action.
Chris DeGarmo + Geoff Tate: Ugh, just thinking about how horrible the lyrics were on the most recent Queensryche record is enough to make me ill, but back in the day, Geoff and Chris were one of the best lyrical teams around.
Andrew Eldritch: Say what you will about the man, but Sisters of Mercy has some pretty intellectual and thought-provoking lyrics in their songs, marking him out as a master of his craft.
Dio: The ultimate heavy metal storyteller.
Also want to mention that Steve Harris, Neil Peart, Freddy Mercury, Warrel Dane and a few others already posted would make my list, but I primarily wanted to give special attention to those I felt deserved to be in this thread but had not yet been named.