You may have to break this off into categories to be fair. But to comment on some of the ones mentioned already:
Steve Perry (somewhat like Dennis DeYoung)....YUK!!! I liked them back then, but not now...sorry. Great range however.
Van Morrison - YES, excellent voice. One of the classics, for sure.
Al Green - Love it, but then don't forget Lou Rawls and Barry White too!!
Justin Hayward - Nice voice, one of my favorites, but I wouldn't put it up with too many of the greats really.
How about David Lee Roth, Bob Seger, Huey Lewis, or George Thoroughgood for good old Rock n Roll?
One of my all-time favorites would have to be Burton Cummings. Very versitile voice.
And rock's all-time "screamer" - Edgar Winter. Just listen to his live version of "Tobacco Road" from the 'Roadwork' album and you'll see what I mean. Excellent song by the way with Edgar, Johnny, and Rick Derringer blasting the blues classic.