Here are some of my favs:
Jon Oliva (Savatage)--Gutter Ballet: Streets is amazing overall, and a lot of it is due to Jon, but I'd have to give Gutter Ballet the nod for a killer vocal performance, particularly in the barnburners such as She's In Love and Unholy.
Steve Souza (Exodus)--Tempo of the Damned: Over a decade after the band's breakup, they return with a fierce statement, and Souza was a big part of that statement. He may not have the range of many of the other vocalists mentioned in this thread, but he definitely has the power and anger to carry those songs. I was very disappointed when Steve left later that year (although at least I got to see them live on that tour), and am very glad he is back.
Hansi Kursch (Demons and Wizards)--Demons and Wizards: His work in Blind Guardian is amazing, but I feel that he really took it to a new level on the debut album of this side project with John Schaffer (Iced Earth). It's a shame that we haven't seen anything else from this project (aside from a 2005 follow-up) because Hansi really worked with with Schaffer.
Mike Howe (Metal Church)--Blessing in Disguise: Many believe that David Wayne was Metal Church, but I believe that his replacement Mike Howe took them to a higher level artistically and vocally. He had the range and the passon. Plus, Anthem to the Estranged is a modern day classic, largely due to the haunting vocals of Mike Howe.
Ronnie James Dio (Mob Rules, Heaven and Hell): I can't pick between those two albums, because both are vocal masterpieces.
Keith Sudano (Eternity X)--Zodiac: This is a highly underrated and overlooked band from the mid 90's, and a lot of it was due to Sudano's quirky vocals. Some liked their latter day stuff, but I believe their first album, a concept album around the signs of the zodiac was their best.
Jack Russell (Great White): Psycho City: He may not have had the range of other singers, but he certainly had the passion, and this was an album where everything came together. Unfortunately as it was released during height of the grunge era, it disappeared pretty quickly, but check out the music and the vocals for a very good performance, particularly on the sultry Old Rose Motel and the quiet Maybe Someday.