Ha! Paul Simon, a big no from me. I heard him sing Bridge Over Troubled Waters and it was so inferior to Art's many renditions. Art is a big yes from me. Remember it's about their voice not songwriting.
Adding a few more: Steve Winwood (how could I forget that wonderful voice?) Kenny Loggins George Michael
A few of my favorites: Myles Kennedy Chris Cornell Sammy Hagar Ronnie James Dio John Fogerty David Coverdale Robert Plant Ann Wilson Not into these singers main music but impressed by their voice: Pink - singing Me and Bobby Mccgee Fergie - singing Barricuda