Modern Bluegrass

Sara Watkins
Sarah Jarosz
Aoife O’Donovan
Mandolin Orange
Stray Birds
Shovels & Rope

I second Billy Strings. 

If you like Nickel Creek have a listen to Sarah Jarosz Follow me Down - great production really good album.

Punch Brothers would likely be a good one to check out as well.

Pretty much anything by David Grisman sounds great and has excellent musicians on it.

Try the Steep Canyon Rangers start with and their album the North Carolina Songbook.

+1    Really good album!

As others have said, check out Billy Strings. This young guy is phenomenal. He plays guitar, banjo, and mandolin; sings; and writes songs. Knows all the classic tunes, and seems to be all about the music. I just love him!
If you haven't seen it, this YouTube of him with Del McCoury is a great, It's hard to say who's more thrilled, the young guy playing with the legend, or the legend playing with the young guy sure to carry the torch forward.