Chuck Schuldinger is as sure as hell not my favorite vocalist! Although he hasn't the worst vocal I've heard, I never liked his vocal, but I have gotten used to it.
Marvin Gaye is my favorite singer of all time, but I don't think he plays guitar. If he does, I don't think I've heard him do it, as he, as far as I know, doesn't play guitar on his records.
Ronald Isley of the Isley Brothers doesn't play guitar either (as far as I know), but he's a great singer!
A favorite guitarist/singer could be Jeff Buckley. I think he's a great singer and I think he's a decent guitarplayer too, but I've grown a little tired of his music, so maybe it would be stupid picking him. Mikael Åkerfeldt is probably the only one I can come up with. I don't really like the growling, although it fits the music pretty good, I think his clean vocals are decent!
As I said in another thread (or maybe it was on the Opeth board) I've never found a singer who screams/growls, that I would call a good singer. Shagrath is one of the "better" (or less worse) of those though. Satyr is probably too.