Well, I'm a Celtoid kinda guy, so I'd say Waterson:Carthy, Patrick Street, Kate Rusby, Altan, Old Blind Dogs, Brian McNeill, Fairport Convention, Deanta, De Danann... any band that has fiddle and vocals. There's much to be learned from listening to how the pros arrange their stuff.

If you're more into the American style, there's John Hartford, Alison Krauss, (early) Emmylou Harris, David Bromberg, er, um... as I said, I'm more of a Celtoid kinda guy.

Cheers,
Michael