Gee, I haven't seen Ricky Skaggs mentioned yet. I have to agree with Jesse McReynolds. Also, John Duffy and Buck White. I haven't yet heard of Simon Mayor, but I'll have to look into him. There's also a local guy here (in Seattle) named Tom Moran who is an absolute phenomenal player. He's very adept at bluegrass as well as jazz and blues. I've had a few lessons with him and he's also a great teacher.