I also have a few favorites (but I'm of course looking for more, hence the post). The obvious ones besides Doc Watson:
Bill Monroe
Tony Rice
Earl Scruggs
Del McCoury
The Dillards
John Denver
Osborne Brothers
Ricky Skaggs
Tony Trischka
A more obscure (at least it was to me) name whose playing really knocked out/surprised me when I first heard it:
Larry Keel