Andre Matos (ex-Angra). I love his voice, but I know a lot of people who don't.
Tony Martin (ex-Black Sabbath). I think a lot of people disliked him on principle. I thought he was a great studio singer, and Headless Cross and Eternal Idol are a couple of my favorite Sabbath albums.
John Arch (ex Fates Warning). I can certainly understand that some people dislike the upper-register singers. I thought he was very melodic and dynamic and is a very big reason why I'm such a huge fan of early Fates Warning.
John Greely (ex-Iced Earth). Sang on "Night of the Stormrider", one of my top three albums of all time, let alone the best thing Iced Earth has ever done.