I will list what my idea is and everyone is welcome to list theirs
Guitar : Tony Rice
Fiddle: Mark O’connor
Dobro :Jerry Douglas
Mandolin : Sam Bush
I know it is a very competitive and it is in the ear of the listener. List your preference it might be interesting. For Bluegrass and folk type those four are my picks.
This could get interesting!
Guitar: Tony Rice
Fiddle: Charlie Daniels
Banjo: Ron Block … I mean Ben Clark
Man, this is tough. Here’s what pops out off the top of my head for bluegrass musicians:
Guitar: Bryan Sutton (although TR and CW were certainly more influential)
Mandolin: Chris Thile (same about influence and Bill Monroe)
Dobro: Jerry Douglas (pretty much any way I slice it… he’s the best I know)
Fiddle: Mark O’Connor (although if you can count the vocals they have, I’d go with Alison)
You’re right Mike this is tough. Therefore I’m going with a few of what I think are tops (not necessarily in order), I just can’t pick one:
Guitar: Tony Rice, Kenny Smith, Bryan Sutton, Courtney Hartman and of course Doc!
Fiddle: Michael Cleveland, Jay Ungar, April Verch
Dobro: Jerry Douglas, Rob Ickes
Mandolin: Adam Steffey, Ronnie McCoury, Jesse Brock, but Chris Thile is unbelievable
Banjo: 
Can’t go there but Earl seemed to make an impact
Bass: I always liked the guy from The Dillards with the pipe
Thanks for this Ken.
Maybe not the best but my personal favorites:
Guitar: Norman Blake
Fiddle: Kenny Baker
Mandolin: Ricky Skaggs (Sierra Hull for the newer generation)
Banjo: Ron Block
Dobro: Josh Graves
Bass: Barry Bales
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Banjo: Can’t go there but Earl seemed to make an impact …
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You could call it that!
Nice Topic!!!
My vote
For Guitar, Doc
Banjo, (Electric) Buck Trent
Mandolin, Chris Thile
Fiddle, Martie Maguire
Narrowing down to one favorite musician for each instrument is just too difficult for me, but I did get to see Blue Highway’s new dobro player, Gaven Largent, perform last week and the kid is a monster already. He doesn’t seem to be intimidated by replacing Rob Ickes at all.
The truth is : no one person can fill the slot of best because we have such a wide group or groups to pick from and personal preference which is what I was looking for and got in spades thank you all for joining in, I enjoyed hearing from each of you .
I guess at this site we’re looking primarily at bluegrass players. I don’t know what best means either, but some of my favorites:
Mandolin: Joe Walsh and Tim O’Brien. Sam Bush and Sarah Jarosch are in the mix too. I actually don’t care much for Thile. I don’t know why, I guess I like to imagine myself playing the stuff that the guys I listen to play, and with Thile that’s too much of a stretch. But also, he’s just too busy, if a phrase needs 2 notes he’ll play 55. I like it less cluttered.
Fiddle: Michael Cleveland is awesome. First time I saw him up our way, I didn’t even realize he was blind. I also dig Stuart Duncan and Alison Krause.
Guitar: Stormin’ Norman. And of course, The Man. I never did get the chance to see Tony live, and it doesn’t sound like I ever will. Sniff.
Dobro: Jerry Douglas, Cindy Cashdollar although I’m not sure how BG she is considered.
Bass: I don’t know, they all sound good, and the same to my ears. The bass player in my band would have my head if he heard me say that.
Banjo: Uh, no. Thanks for asking. 
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Banjo: Uh, no. Thanks for asking.
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'splain yourself.
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Banjo: Uh, no. Thanks for asking.
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'splain yourself.
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Uh-oh. Okay I’ll go with Ron Block. And I’m not just saying that because you did, or because he typically only plays it on a handful of songs on each Union Station album. As my Thile comments suggested, I’m big on restraint and moderation. These are not generally considered strong suits of the banjo, but Ron does keep it down to a dull roar. 
I did not pick a fav on Banjo as I always think of Scruggs as the top player I guess I figured everyone did . He got it started for sure . they all sound great and yes I was referring to Bluegrass music and folk to . Thanks to all who posted here .
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Banjo: Uh, no. Thanks for asking.
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'splain yourself.
Uh-oh. Okay I’ll go with Ron Block. And I’m not just saying that because you did, or because he typically only plays it on a handful of songs on each Union Station album. As my Thile comments suggested, I’m big on restraint and moderation. These are not generally considered strong suits of the banjo, but Ron does keep it down to a dull roar.
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Touché 
You should listen to Steve Martin’s The Crow for some restrained banjoing. My wife - who bought me a banjo mute - is even a fan of that album.
Steve Martin is actually up in Boston for an extended stay. He’s a guest curator at the Museum of Fine Arts. There is a fair sized BG community in the area, so I’ll have to keep an eye out to see if he does any shows with his band while he’s here.
Late to the debate.
Guitar: Doc, Tony (in his younger days), and these days David Grier and Bryan Sutton.
Fiddle: Michael Cleveland, Sara Watkins (extremely tasteful), Mark O’Connor.
Banjo: Earl (of course) and I am digging Keith Agnes on from Country Current.
Mandolin : So much talent in this category. Chris Thiele has to be on top with rock solid players like Ricky Skaggs and Adam Steffey.
Dobro: Is locked down by Jerry Douglas but there are some fine a fun players out there like Chris Hart from the Bluegrass Brothers.
However, if we are picking (pun intended) favorites, then favorite band which has a combination of great playing, high energy and killer harmonies should be included.
Band: Kentucky Thunder would have to be near the top of this list for me with Mountain Heart, Nickel Creek and Cherryholmes stepping in between sets.
Since Doc opened up another category I’ll chime back in. Favorite band would also be Kentucky Thunder with The Rage (Rhonda Vincent) being a close second. Was fortunate to see Ricky and his band in concert a year or two ago and will see Rhonda in concert a week from Saturday!
I agree with all mentioned.
Bands brought to mind J.D Crowe and The Bluegrass Album Band. That in turn brought J.D. Crowe to mind for one of the top banjo players. I’m not sure if he was mentioned.
For more modern bands, I would have to add Balsam Range to the list.
Bands: If I voted with my purchases, I have more Alison Kraus/Union Station CDs than any other group. Second in that count is KY Thunder.
New category: Favorite “album”… First that comes to mind is Skaggs and Rice. With that said, I wore out AKUS’s “So long so wrong”. Favorite Ky Thunder one might be “History of the future.” “Mother’s only sleeping” gets to me everytime. The Purple Hull’s “Close to home” got played a ton as well. WIth all that said, my mixtapes of various groups get more play than any particular band.
I just thought of someone else to be in the conversation for fiddle (I don’t think he’s come up yet)… Andy Leftwich. He’s pretty amazing and he still has much music to be made in the future.