I'm glad someone mentioned Rick Danko. I think he has one of the greatest voices ever. Listening to It Makes No Difference fro The Last Waltz give me goosebumps every single fookin' time.
And on bass he's no slough either. No virtuoso shit, but always the right notes and very melodic. Listen to his great fretless Ampeg Scroll Bass parts on Rock of Ages or of course his fretted Gibson Ripper work on The Last Waltz.
And if you're doin' this (you should!!!!) also take a listen to The Weight with the Staples also from The Last Waltz. If the Danko singing part comes on his timing is so unique he almost changes the whole song. I just love the guy.
And a great singer/bass player: Ronnie 'Plonk' Lane of Faces and Small Faces. Go to YouTube for a 72 live version of Macca's Maybe I'm Amazed by The Faces. Ronnie starts the song, Rod the Mod takes over and they do a part together. Great bass sound too!