For my money Eddie Lang is the greatest accompanist ever. Not primarily a folkie obviously, he was a jazz guitarist, though he also recorded loads of folk(blues) material. As an accompanist he was absolutely superb, whether for fellow guitarist Lonnie Johnson, many singers(Bing Crosby on numerous records) and of course all the jazz band work. As a sample, have a listen to Bix Beiderbecke's "I'm Coming Virginia". The recording is a good lesson in Eddie Lang's accompanyingskills as he is put right up front in the mix so you can enjoy all his twiddly bits(and he's featured in the intro). Bix doesn't come in till half way through, by the way, so don't sit thinking "Where's the trumpet?"
Basically, people accompanied by Eddie Lang always sounded better than they otherwise would have: that's the test.