Groundbreaking guitarists: Eddie Van Halen, BB King, Jimi Hendrix
Groundbreaking drummers: Buddy Guy, Louis Bellson(good friend R.I.P), Neil Peart(a canadian but this side anyway)
Groundbreaking bass players: Les Claypool, Mike Balzary(flea from rhcp)
Not to mention Louis Armstrong, Miles Davis, Winton Marsalis

given the devil his due.. i can't think of a single singer I'd put on my list higher than Freddie Mercury.  Still makes me sorta sad that he's gone. 

The list of overly talented American musicians is too numerous to really give full credit in any forum; but, the list would be incomplete without my best friend Eric Kranz.  Probably one of the best drummers I've ever seen in person and I used to work at a jazz bar who was had Louis Bellson(youtube him) as a sitting drummer.  Equally talented on bass, guitar, just about every brass instrument and nearly every woodwind you can think of.  One of those rare guys who can just easily figure out how to make music come out of anything.  And the moron is toiling away in obscurity as a sound engineer and p.a. designer in western illinois.