I'm just going to list a few favorites here
Grateful Dead - I missed Jerry, but I've seen the rest of the boys many times. It's ironic how this icon of hippie counterculture has such a deep connection to American roots music.
John Hartford - Guys, get Steam Powered Aereo-Plane. Seriously.
Charlie Mingus - Gotta love a jazz band fronted by the bassist.
Sound Tribe Sector 9 - Probably my favorite act to see at a festival these days.
Nine Inch Nails - This was High School for me, finally saw him last year. Fucking tore it up.
Parliament/Funkadelic - P. Funk parties really don't stop. See them at a festival if you can.
David Grisman - Most innovative mandolin player since Bill Monroe.
Strength in Numbers - Bluegrass supergroup, every member is a legend in his own right. Listen
here, you'll thank me. Very modern stuff.
Phish - Damn I wish I could have gotten tickets this year.
Bill Monroe - The master.
J.S. Bach - I love his fugues, and the solo cello and violin suites. Amazing polyphonies even from a single instrument.
Mike Seeger - Great old-time performer.
I tried last.fm, but I have so much music it's rare for anything to get listened to more than twice. So it can't really generate meaningful statistics.
I used to hate country, and I still hate George Jones, Garth Brooks, etc. But there's a lot more to country than that. There's a rich tradition of amazing musicianship. Give bluegrass a try. Steam Powered Areo-Plane is *the* bridge between traditional bluegrass and somewhat jazzier "newgrass". It's a great place to start.