This looks like it could run and run. I'd also choose Burl Ives, Johnny Cash & Harry Belafonte.

I saw Ewan MacColl on stage at Stratford East before I first heard him sing at Soho Square. The theatrical background was always evident in his singing.

Extending the style boundaries a little, how about Paul Robeson. Best in his stage roles, although some of the films were not bad, and his singing was top-class.

A current favourite is John Telfer. A few extracts from a BBC biog when he joined the cast of "The Archers":

"He went on to study at the Bristol Old Vic Theatre School and has appeared in more than 35 plays with the Bristol Old Vic Company.

These include Titus Andronicus, The Golden Pathway Annual, Comedians, All My Sons, A Midsummer Night's Dream – which transferred to The Old Vic London – and most recently the premiere of Black/White. He played Mortimer and Westmoreland in the recent Henry IV parts 1 and 2."

He has written music for stage, television and radio, and a CD of his songs and occasional music for Shakespeare at the Tobacco Factory productions is now available.

John also fronts a big band, singing standards, and is lead singer and keyboard player with rock band afgm (Afewgoodmen). The band released their third album in October 2004 and are looking for a record deal."

Full biog here:
John Telfer biog

The only thing I don't like about John is that he was the dream man of both my wife and my daughter in the early 80's. Glad to say they've both gone off him now.

Phil